Rename the [Ee]xts dir to ext (#38852)

**WARNING:** This PR will probably blow up all in-flight PRs

at some point in the early days of CmdPal, two of us created seperate
`Exts` and `exts` dirs. Depending on what the casing was on the branch
that you checked one of those out from, it'd get stuck like that on your
PC forever.

Windows didn't care, so we never noticed.

But GitHub does care, and now browsing the source on GitHub is basically
impossible.

Closes #38081

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Calc/CalculatorCommandProvider.cs

@@ -0,0 +1,177 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Collections.Generic;
+using System.Data;
+using System.Globalization;
+using System.Text;
+using Microsoft.CmdPal.Ext.Calc.Properties;
+using Microsoft.CommandPalette.Extensions;
+using Microsoft.CommandPalette.Extensions.Toolkit;
+using Windows.Foundation;
+
+namespace Microsoft.CmdPal.Ext.Calc;
+
+public partial class CalculatorCommandProvider : CommandProvider
+{
+    private readonly ListItem _listItem = new(new CalculatorListPage()) { Subtitle = Resources.calculator_top_level_subtitle };
+    private readonly FallbackCalculatorItem _fallback = new();
+
+    public CalculatorCommandProvider()
+    {
+        Id = "Calculator";
+        DisplayName = Resources.calculator_display_name;
+        Icon = IconHelpers.FromRelativePath("Assets\\Calculator.svg");
+    }
+
+    public override ICommandItem[] TopLevelCommands() => [_listItem];
+
+    public override IFallbackCommandItem[] FallbackCommands() => [_fallback];
+}
+
+// The calculator page is a dynamic list page
+// * The first command is where we display the results. Title=result, Subtitle=query
+//   - The default command is `SaveCommand`.
+//     - When you save, insert into list at spot 1
+//     - change SearchText to the result
+//   - MoreCommands: a single `CopyCommand` to copy the result to the clipboard
+// * The rest of the items are previously saved results
+//   - Command is a CopyCommand
+//   - Each item also sets the TextToSuggest to the result
+[System.Diagnostics.CodeAnalysis.SuppressMessage("StyleCop.CSharp.MaintainabilityRules", "SA1402:File may only contain a single type", Justification = "This is sample code")]
+public sealed partial class CalculatorListPage : DynamicListPage
+{
+    private readonly List<ListItem> _items = [];
+    private readonly SaveCommand _saveCommand = new();
+    private readonly CopyTextCommand _copyContextCommand;
+    private readonly CommandContextItem _copyContextMenuItem;
+    private static readonly CompositeFormat ErrorMessage = System.Text.CompositeFormat.Parse(Properties.Resources.calculator_error);
+
+    public CalculatorListPage()
+    {
+        Icon = IconHelpers.FromRelativePath("Assets\\Calculator.svg");
+        Name = Resources.calculator_title;
+        PlaceholderText = Resources.calculator_placeholder_text;
+        Id = "com.microsoft.cmdpal.calculator";
+
+        _copyContextCommand = new CopyTextCommand(string.Empty);
+        _copyContextMenuItem = new CommandContextItem(_copyContextCommand);
+
+        _items.Add(new(_saveCommand) { Icon = new IconInfo("\uE94E") });
+
+        UpdateSearchText(string.Empty, string.Empty);
+
+        _saveCommand.SaveRequested += HandleSave;
+    }
+
+    private void HandleSave(object sender, object args)
+    {
+        var lastResult = _items[0].Title;
+        if (!string.IsNullOrEmpty(lastResult))
+        {
+            var li = new ListItem(new CopyTextCommand(lastResult))
+            {
+                Title = _items[0].Title,
+                Subtitle = _items[0].Subtitle,
+                TextToSuggest = lastResult,
+            };
+            _items.Insert(1, li);
+            _items[0].Subtitle = string.Empty;
+            SearchText = lastResult;
+            this.RaiseItemsChanged(this._items.Count);
+        }
+    }
+
+    public override void UpdateSearchText(string oldSearch, string newSearch)
+    {
+        var firstItem = _items[0];
+        if (string.IsNullOrEmpty(newSearch))
+        {
+            firstItem.Title = Resources.calculator_placeholder_text;
+            firstItem.Subtitle = string.Empty;
+            firstItem.MoreCommands = [];
+        }
+        else
+        {
+            _copyContextCommand.Text = ParseQuery(newSearch, out var result) ? result : string.Empty;
+            firstItem.Title = result;
+            firstItem.Subtitle = newSearch;
+            firstItem.MoreCommands = [_copyContextMenuItem];
+        }
+    }
+
+    internal static bool ParseQuery(string equation, out string result)
+    {
+        try
+        {
+            var resultNumber = new DataTable().Compute(equation, null);
+            result = resultNumber.ToString() ?? string.Empty;
+            return true;
+        }
+        catch (Exception e)
+        {
+            result = string.Format(CultureInfo.CurrentCulture, ErrorMessage, e.Message);
+            return false;
+        }
+    }
+
+    public override IListItem[] GetItems() => _items.ToArray();
+}
+
+[System.Diagnostics.CodeAnalysis.SuppressMessage("StyleCop.CSharp.MaintainabilityRules", "SA1402:File may only contain a single type", Justification = "This is sample code")]
+public sealed partial class SaveCommand : InvokableCommand
+{
+    public event TypedEventHandler<object, object> SaveRequested;
+
+    public SaveCommand()
+    {
+        Name = Resources.calculator_save_command_name;
+    }
+
+    public override ICommandResult Invoke()
+    {
+        SaveRequested?.Invoke(this, this);
+        return CommandResult.KeepOpen();
+    }
+}
+
+[System.Diagnostics.CodeAnalysis.SuppressMessage("StyleCop.CSharp.MaintainabilityRules", "SA1402:File may only contain a single type", Justification = "This is sample code")]
+internal sealed partial class FallbackCalculatorItem : FallbackCommandItem
+{
+    private readonly CopyTextCommand _copyCommand = new(string.Empty);
+    private static readonly IconInfo _cachedIcon = IconHelpers.FromRelativePath("Assets\\Calculator.svg");
+
+    public FallbackCalculatorItem()
+        : base(new NoOpCommand(), Resources.calculator_title)
+    {
+        Command = _copyCommand;
+        _copyCommand.Name = string.Empty;
+        Title = string.Empty;
+        Subtitle = Resources.calculator_placeholder_text;
+        Icon = _cachedIcon;
+    }
+
+    public override void UpdateQuery(string query)
+    {
+        if (CalculatorListPage.ParseQuery(query, out var result))
+        {
+            _copyCommand.Text = result;
+            _copyCommand.Name = string.IsNullOrWhiteSpace(query) ? string.Empty : Resources.calculator_copy_command_name;
+            Title = result;
+
+            // we have to make the subtitle the equation,
+            // so that we will still string match the original query
+            // Otherwise, something like 1+2 will have a title of "3" and not match
+            Subtitle = query;
+        }
+        else
+        {
+            _copyCommand.Text = string.Empty;
+            _copyCommand.Name = string.Empty;
+            Title = string.Empty;
+            Subtitle = string.Empty;
+        }
+    }
+}