Add analyzer for [field: ObservableProperty] uses from auto-properties

Closes #734

This PR adds a new analyzer to emit a diagnostic when using [field: ObservableProperty] from an auto-property.

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] New component
- [ ] Pull Request has been submitted to the documentation repository instructions. Link:
[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