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.UI/Pages/ShellPage.xaml.cs

@@ -171,7 +171,7 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
     // This gets called from the UI thread
     private async Task HandleConfirmArgsOnUiThread(IConfirmationArgs? args)
     {
-        if (args == null)
+        if (args is null)
         {
             return;
         }
@@ -236,7 +236,7 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
 
     public void OpenSettings()
     {
-        if (_settingsWindow == null)
+        if (_settingsWindow is null)
         {
             _settingsWindow = new SettingsWindow();
         }
@@ -324,7 +324,7 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
                 // command from our list of toplevel commands.
                 var tlcManager = App.Current.Services.GetService<TopLevelCommandManager>()!;
                 var topLevelCommand = tlcManager.LookupCommand(commandId);
-                if (topLevelCommand != null)
+                if (topLevelCommand is not null)
                 {
                     var command = topLevelCommand.CommandViewModel.Model.Unsafe;
                     var isPage = command is not IInvokableCommand;