CmdPal: Add settings to control which fallbacks are enabled (#40505)

This adds settings to each provider to allow us to control if individual
fallback items are enabled or not, regardless of the provider being
enabled.

This is relevant to _all the threads where disabling fallback commands
came up_

This just adds another section to each provider's settings page, with a
list of the fallback commands.

This also has nothing to do with the "top-level apps search", which is
not really a fallback command - it's its own thing.

Ref #38288. Doesn't close that, because this only controls
enable/disable, not ranking.

From here, we should be able to add a dedicated page in the SUI that
shows all the fallbacks across all providers. That's where we'll enable
the ordering.

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Indexer/FallbackOpenFileItem.cs

@@ -15,6 +15,8 @@ namespace Microsoft.CmdPal.Ext.Indexer;
 
 internal sealed partial class FallbackOpenFileItem : FallbackCommandItem, System.IDisposable
 {
+    private static readonly NoOpCommand _baseCommandWithId = new() { Id = "com.microsoft.indexer.fallback" };
+
     private readonly CompositeFormat fallbackItemSearchPageTitleCompositeFormat = CompositeFormat.Parse(Resources.Indexer_fallback_searchPage_title);
 
     private readonly SearchEngine _searchEngine = new();
@@ -22,10 +24,11 @@ internal sealed partial class FallbackOpenFileItem : FallbackCommandItem, System
     private uint _queryCookie = 10;
 
     public FallbackOpenFileItem()
-        : base(new NoOpCommand(), Resources.Indexer_Find_Path_fallback_display_title)
+        : base(_baseCommandWithId, Resources.Indexer_Find_Path_fallback_display_title)
     {
         Title = string.Empty;
         Subtitle = string.Empty;
+        Icon = Icons.FileExplorer;
     }
 
     public override void UpdateQuery(string query)

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Indexer/Pages/ActionsListContextItem.cs

@@ -6,7 +6,6 @@ using System;
 using System.Collections.Generic;
 using System.Globalization;
 using System.Threading;
-using System.Threading.Tasks;
 using ManagedCommon;
 using Microsoft.CmdPal.Ext.Indexer.Commands;
 using Microsoft.CmdPal.Ext.Indexer.Data;