CmdPal: Null pattern matching based on is expression rather than overridable operators (#40972)

What the title says. 😄 

Rather than relying on the potentially overloaded `!=` or `==` operators
when checking for null, now we'll use the `is` expression (possibly
combined with the `not` operator) to ensure correct checking. Probably
overkill for many of these classes, but decided to err on the side of
consistency. Would matter more on classes that may be inherited or
extended.

Using `is` and `is not` will provide us a guarantee that no
user-overloaded equality operators (`==`/`!=`) is invoked when a
`expression is null` is evaluated.

In code form, changed all instances of:

```c#
something != null

something == null
```

to:

```c#
something is not null

something is null
```

The one exception was checking null on a `KeyChord`. `KeyChord` is a
struct which is never null so VS will raise an error when trying this
versus just providing a warning when using `keyChord != null`. In
reality, we shouldn't do this check because it can't ever be null. In
the case of a `KeyChord` it **would** be a `KeyChord` equivalent to:

```c#
KeyChord keyChord = new ()
{
    Modifiers = 0,
    Vkey = 0,
    ScanCode = 0
};
```

src/modules/cmdpal/Microsoft.CmdPal.Core.ViewModels/CommandBarViewModel.cs

@@ -2,7 +2,6 @@
 // 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.Collections.ObjectModel;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Messaging;
 using Microsoft.CmdPal.Core.ViewModels.Messages;
@@ -35,13 +34,13 @@ public partial class CommandBarViewModel : ObservableObject,
     [NotifyPropertyChangedFor(nameof(HasPrimaryCommand))]
     public partial CommandItemViewModel? PrimaryCommand { get; set; }
 
-    public bool HasPrimaryCommand => PrimaryCommand != null && PrimaryCommand.ShouldBeVisible;
+    public bool HasPrimaryCommand => PrimaryCommand is not null && PrimaryCommand.ShouldBeVisible;
 
     [ObservableProperty]
     [NotifyPropertyChangedFor(nameof(HasSecondaryCommand))]
     public partial CommandItemViewModel? SecondaryCommand { get; set; }
 
-    public bool HasSecondaryCommand => SecondaryCommand != null;
+    public bool HasSecondaryCommand => SecondaryCommand is not null;
 
     [ObservableProperty]
     public partial bool ShouldShowContextMenu { get; set; } = false;
@@ -58,14 +57,14 @@ public partial class CommandBarViewModel : ObservableObject,
 
     private void SetSelectedItem(ICommandBarContext? value)
     {
-        if (value != null)
+        if (value is not null)
         {
             PrimaryCommand = value.PrimaryCommand;
             value.PropertyChanged += SelectedItemPropertyChanged;
         }
         else
         {
-            if (SelectedItem != null)
+            if (SelectedItem is not null)
             {
                 SelectedItem.PropertyChanged -= SelectedItemPropertyChanged;
             }
@@ -88,7 +87,7 @@ public partial class CommandBarViewModel : ObservableObject,
 
     private void UpdateContextItems()
     {
-        if (SelectedItem == null)
+        if (SelectedItem is null)
         {
             SecondaryCommand = null;
             ShouldShowContextMenu = false;
@@ -127,13 +126,13 @@ public partial class CommandBarViewModel : ObservableObject,
     public ContextKeybindingResult CheckKeybinding(bool ctrl, bool alt, bool shift, bool win, VirtualKey key)
     {
         var keybindings = SelectedItem?.Keybindings();
-        if (keybindings != null)
+        if (keybindings is not null)
         {
             // Does the pressed key match any of the keybindings?
             var pressedKeyChord = KeyChordHelpers.FromModifiers(ctrl, alt, shift, win, key, 0);
             if (keybindings.TryGetValue(pressedKeyChord, out var matchedItem))
             {
-                return matchedItem != null ? PerformCommand(matchedItem) : ContextKeybindingResult.Unhandled;
+                return matchedItem is not null ? PerformCommand(matchedItem) : ContextKeybindingResult.Unhandled;
             }
         }
 
@@ -142,7 +141,7 @@ public partial class CommandBarViewModel : ObservableObject,
 
     private ContextKeybindingResult PerformCommand(CommandItemViewModel? command)
     {
-        if (command == null)
+        if (command is null)
         {
             return ContextKeybindingResult.Unhandled;
         }