CmdPal: Remove dead code from Window Walker and cleanup (#45570)

## Summary of the Pull Request

This PR is a light cleanup on Window Walker built-in extension:

- Removes unused types
- Removes redundant code
- Fixes inconsistent naming conventions
- De-LINQ-ify
- Fixes XML doc
- Updates language constructs to the latest (use of
System.Threading.Lock, Collection expressions)
- Updates types and members visibility and makes static classes static

⚠️ Trickiest part are 44ac1c1 and 26c946c that removes redundant null
checks.

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.WindowWalker/Helpers/ProcessPackagingInspector.cs

@@ -2,9 +2,7 @@
 // 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;
 using System.Runtime.InteropServices;
-using System.Text;
 using Microsoft.Win32.SafeHandles;
 
 namespace Microsoft.CmdPal.Ext.WindowWalker.Helpers;