Add new analyzer/fixer for async void [RelayCommand] methods

[Sergio0694]

Description

This PR adds a new analyzer and code fixer for async void returning [RelayCommand] methods.

PR Checklist

• [X] Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• [X] Based off latest main branch of toolkit
• [X] PR doesn't include merge commits (always rebase on top of our main, if needed)
• [X] Tested code with current supported SDKs
• [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
• [X] Header has been added to all new source files (run build/UpdateHeaders.bat)
• [X] Contains NO breaking changes
• [X] Every new API (including internal ones) has full XML docs
• [X] Code follows all style conventions

[Sergio0694]

cc. @Youssef1313 as usual in case you felt in the mood to double check some analyzers/fixers 😄