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.ViewModels/Commands/CreatedExtensionForm.cs

@@ -30,7 +30,7 @@ internal sealed partial class CreatedExtensionForm : NewExtensionFormBase
     public override ICommandResult SubmitForm(string inputs, string data)
     {
         var dataInput = JsonNode.Parse(data)?.AsObject();
-        if (dataInput == null)
+        if (dataInput is null)
         {
             return CommandResult.KeepOpen();
         }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/LogMessagesPage.cs

@@ -23,7 +23,7 @@ public partial class LogMessagesPage : ListPage
 
     private void LogMessages_CollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
     {
-        if (e.Action == NotifyCollectionChangedAction.Add && e.NewItems != null)
+        if (e.Action == NotifyCollectionChangedAction.Add && e.NewItems is not null)
         {
             foreach (var item in e.NewItems)
             {

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/MainListPage.cs

@@ -203,7 +203,7 @@ public partial class MainListPage : DynamicListPage,
 
             // If we don't have any previous filter results to work with, start
             // with a list of all our commands & apps.
-            if (_filteredItems == null)
+            if (_filteredItems is null)
             {
                 _filteredItems = commands;
                 _filteredItemsIncludesApps = _includeApps;

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/NewExtensionForm.cs

@@ -98,7 +98,7 @@ internal sealed partial class NewExtensionForm : NewExtensionFormBase
     public override CommandResult SubmitForm(string payload)
     {
         var formInput = JsonNode.Parse(payload)?.AsObject();
-        if (formInput == null)
+        if (formInput is null)
         {
             return CommandResult.KeepOpen();
         }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/NewExtensionPage.cs

@@ -14,7 +14,7 @@ public partial class NewExtensionPage : ContentPage
 
     public override IContent[] GetContent()
     {
-        return _resultForm != null ? [_resultForm] : [_inputForm];
+        return _resultForm is not null ? [_resultForm] : [_inputForm];
     }
 
     public NewExtensionPage()
@@ -28,13 +28,13 @@ public partial class NewExtensionPage : ContentPage
 
     private void FormSubmitted(NewExtensionFormBase sender, NewExtensionFormBase? args)
     {
-        if (_resultForm != null)
+        if (_resultForm is not null)
         {
             _resultForm.FormSubmitted -= FormSubmitted;
         }
 
         _resultForm = args;
-        if (_resultForm != null)
+        if (_resultForm is not null)
         {
             _resultForm.FormSubmitted += FormSubmitted;
         }