Follow up for #3206

PR Type

What kind of change does this PR introduce?

• Bugfix
• Code style update (formatting)
• Refactoring (no functional changes, no api changes)
• Optimization

What is the new behavior?

This PR includes a number of optimizations, tweaks and refactorings to DispatcherQueueHelper:

• Removed unnecessary null checks for function, and enabled nullability annotations. Those null checks were unnecessary since there were also other parameters that could also be null (ie. the actual DispatcherQueue) anyway, plus we'd still get an exception anyway when those instances are accessed. Instead, we now have nullability annotations to make the intent clearer.
• Avoided the slowdown and memory allocations in case the thread already had access to the dispatcher queue, which was caused by the C# compiler (sadly) always preemptively instantiating the display class for the closure of the lambda expression used in the other branch (I made a sharplab repro here that illustrates the issue). Instead now the fallback path is moved to a separate method, so that the IL has the correct codegen.
• Renamed the class to DispatcherQueueExtensions, since these are all extensions and not a helper class
• Similarly, moved the class to the .Extensions namespace
• Renamed the APIs to EnqueueAsync to mirror the TryEnqueue name of the actual API in the DispatcherQueue class. This better follows the naming convention of the class, and it's also clearer as in case users have multiple windows and different dispatching queues, there isn't really a single "UI thread" - the API is literally just enqueueing an operation on a given dispatcher queue.
• Added a check in case TryEnqueue fails, so if that happens we now return a wrapped InvalidOperationException in the Task, whereas the previous behavior would've just caused that task to never be completed, leaving the caller just waiting forever.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes