Skip field reference diagnostics from constructors

[Sergio0694]

This PR is a small follow up for #532. Specifically it solves issues such as this:

partial class ViewModel
{
    private string name;

    public ViewModel()
    {
        name = ""; // Warning
    }
}

The issue being that:

• If you assign the field, the MVVM Toolkit issues diagnostics
• If you assign the property, you get a nullability warning on the field

The fix is to just not emit diagnostics when assigning fields if the assignment is from a constructor of that type.

cc. @Youssef1313 would you mind also taking a look to confirm if this makes sense? Thank you! 😄

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

[Youssef1313]

Makes sense to me.

[Sergio0694]

Awesome, thank you for taking a look! 😊