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.Shell/Commands/ExecuteItem.cs

@@ -46,7 +46,7 @@ internal sealed partial class ExecuteItem : InvokableCommand
 
     private void Execute(Func<ProcessStartInfo, Process?> startProcess, ProcessStartInfo info)
     {
-        if (startProcess == null)
+        if (startProcess is null)
         {
             return;
         }

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Shell/Helpers/ShellListPageHelpers.cs

@@ -126,7 +126,7 @@ public class ShellListPageHelpers
             return null;
         }
 
-        if (li != null)
+        if (li is not null)
         {
             li.TextToSuggest = searchText;
         }

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Shell/Pages/RunExeItem.cs

@@ -78,7 +78,7 @@ internal sealed partial class RunExeItem : ListItem
         try
         {
             var stream = await ThumbnailHelper.GetThumbnail(FullExePath);
-            if (stream != null)
+            if (stream is not null)
             {
                 var data = new IconData(RandomAccessStreamReference.CreateFromStream(stream));
                 icon = new IconInfo(data, data);

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Shell/Pages/ShellListPage.cs

@@ -245,14 +245,14 @@ internal sealed partial class ShellListPage : DynamicListPage, IDisposable
         var histItemsNotInSearch =
             _historyItems
                 .Where(kv => !kv.Key.Equals(newSearch, StringComparison.OrdinalIgnoreCase));
-        if (_exeItem != null)
+        if (_exeItem is not null)
         {
             // If we have an exe item, we want to remove it from the history items
             histItemsNotInSearch = histItemsNotInSearch
                 .Where(kv => !kv.Value.Title.Equals(_exeItem.Title, StringComparison.OrdinalIgnoreCase));
         }
 
-        if (_uriItem != null)
+        if (_uriItem is not null)
         {
             // If we have an uri item, we want to remove it from the history items
             histItemsNotInSearch = histItemsNotInSearch
@@ -307,8 +307,8 @@ internal sealed partial class ShellListPage : DynamicListPage, IDisposable
         }
 
         var filteredTopLevel = ListHelpers.FilterList(_topLevelItems, SearchText);
-        List<ListItem> uriItems = _uriItem != null ? [_uriItem] : [];
-        List<ListItem> exeItems = _exeItem != null ? [_exeItem] : [];
+        List<ListItem> uriItems = _uriItem is not null ? [_uriItem] : [];
+        List<ListItem> exeItems = _exeItem is not null ? [_exeItem] : [];
 
         return
             exeItems
@@ -459,7 +459,7 @@ internal sealed partial class ShellListPage : DynamicListPage, IDisposable
         var hist = _historyService.GetRunHistory();
         var histItems = hist
             .Select(h => (h, ShellListPageHelpers.ListItemForCommandString(h, AddToHistory)))
-            .Where(tuple => tuple.Item2 != null)
+            .Where(tuple => tuple.Item2 is not null)
             .Select(tuple => (tuple.h, tuple.Item2!))
             .ToList();
         _historyItems.Clear();

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Shell/PathListItem.cs

@@ -72,7 +72,7 @@ internal sealed partial class PathListItem : ListItem
         _icon = new Lazy<IconInfo>(() =>
         {
             var iconStream = ThumbnailHelper.GetThumbnail(path).Result;
-            var icon = iconStream != null ? IconInfo.FromStream(iconStream) :
+            var icon = iconStream is not null ? IconInfo.FromStream(iconStream) :
              _isDirectory ? Icons.FolderIcon : Icons.RunV2Icon;
             return icon;
         });