Relay command sg field keywords

[Avid29]

**Closes #1073 **

When the C# language version supports it, this change replaces the backing field for generated RelayCommands with an implicit field using the new field keyword.

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
• 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

Other information

Technically, this is a breaking change since someone may have been using the backing fields for whatever reason

[Avid29]

Sorry. This PR is horribly named, but GitHub isn't letting me edit for some reason

[Avid29]

Some tests are failing. It appears to be because they test that both the field and property is generated, except now a field is no longer always generated.

For example: Test_ObservableProperty_CommandNamesThatCantBeLowered and Test_RelayCommandAttribute_WithPartialCommandMethodDefinitions both fit this group of tests