Closes #555

This PR updates the [ObservableProperty] generator to support partial properties as well.
It includes the following changes:

• Update the target of all attribute types to support properties too
• Update generator to handle partial properties
• Update all diagnostic messages to also mention partial properties
• Add a new analyzer to error if C# is not set to preview
• Add a new analyzer to error if using a version of Roslyn that's too old
• Add a new analyzer to error if a partial property declaration is invalid
• Add a new analyzer to suggest converting to a partial property when possible
• Add a new code fixer to automatically update all fields to partial properties
• Add unit tests for all new features

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• 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
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions