Add and use new EquatableArray<T> type in source generators

[Sergio0694]

This PR adds a new EquatableArray<T> type and uses it to greatly simplify all source generator incremental models.

See: dotnet/runtime#77183 (comment).

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

[Sergio0694]

Merging this, as it's just the same pattern also used in ComputeSharp and other generators too at this point 🙂