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/ext/Microsoft.CmdPal.Ext.Bookmark/AddBookmarkForm.cs

@@ -63,7 +63,7 @@ internal sealed partial class AddBookmarkForm : FormContent
     public override CommandResult SubmitForm(string payload)
     {
         var formInput = JsonNode.Parse(payload);
-        if (formInput == null)
+        if (formInput is null)
         {
             return CommandResult.GoHome();
         }

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Bookmark/BookmarkPlaceholderForm.cs

@@ -73,7 +73,7 @@ internal sealed partial class BookmarkPlaceholderForm : FormContent
         // parse the submitted JSON and then open the link
         var formInput = JsonNode.Parse(payload);
         var formObject = formInput?.AsObject();
-        if (formObject == null)
+        if (formObject is null)
         {
             return CommandResult.GoHome();
         }

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Bookmark/BookmarksCommandProvider.cs

@@ -49,7 +49,7 @@ public partial class BookmarksCommandProvider : CommandProvider
 
     private void SaveAndUpdateCommands()
     {
-        if (_bookmarks != null)
+        if (_bookmarks is not null)
         {
             var jsonPath = BookmarksCommandProvider.StateJsonPath();
             Bookmarks.WriteToFile(jsonPath, _bookmarks);
@@ -64,12 +64,12 @@ public partial class BookmarksCommandProvider : CommandProvider
         List<CommandItem> collected = [];
         collected.Add(new CommandItem(_addNewCommand));
 
-        if (_bookmarks == null)
+        if (_bookmarks is null)
         {
             LoadBookmarksFromFile();
         }
 
-        if (_bookmarks != null)
+        if (_bookmarks is not null)
         {
             collected.AddRange(_bookmarks.Data.Select(BookmarkToCommandItem));
         }
@@ -93,7 +93,7 @@ public partial class BookmarksCommandProvider : CommandProvider
             Logger.LogError(ex.Message);
         }
 
-        if (_bookmarks == null)
+        if (_bookmarks is null)
         {
             _bookmarks = new();
         }
@@ -134,7 +134,7 @@ public partial class BookmarksCommandProvider : CommandProvider
             name: Resources.bookmarks_delete_name,
             action: () =>
             {
-                if (_bookmarks != null)
+                if (_bookmarks is not null)
                 {
                     ExtensionHost.LogMessage($"Deleting bookmark ({bookmark.Name},{bookmark.Bookmark})");
 

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Bookmark/UrlCommand.cs

@@ -73,7 +73,7 @@ public partial class UrlCommand : InvokableCommand
         if (string.IsNullOrEmpty(args))
         {
             var uri = GetUri(exe);
-            if (uri != null)
+            if (uri is not null)
             {
                 _ = Launcher.LaunchUriAsync(uri);
             }
@@ -109,7 +109,7 @@ public partial class UrlCommand : InvokableCommand
         // First, try to get the icon from the thumbnail helper
         // This works for local files and folders
         icon = await MaybeGetIconForPath(target);
-        if (icon != null)
+        if (icon is not null)
         {
             return icon;
         }
@@ -142,7 +142,7 @@ public partial class UrlCommand : InvokableCommand
         {
             // If the executable exists, try to get the icon from the file
             icon = await MaybeGetIconForPath(fullExePath);
-            if (icon != null)
+            if (icon is not null)
             {
                 return icon;
             }
@@ -154,7 +154,7 @@ public partial class UrlCommand : InvokableCommand
         try
         {
             var uri = GetUri(baseString);
-            if (uri != null)
+            if (uri is not null)
             {
                 var hostname = uri.Host;
                 var faviconUrl = $"{uri.Scheme}://{hostname}/favicon.ico";
@@ -176,7 +176,7 @@ public partial class UrlCommand : InvokableCommand
         try
         {
             var stream = await ThumbnailHelper.GetThumbnail(target);
-            if (stream != null)
+            if (stream is not null)
             {
                 var data = new IconData(RandomAccessStreamReference.CreateFromStream(stream));
                 return new IconInfo(data, data);