More MVVM Toolkit generator improvements

[Sergio0694]

This PR includes a few optimizations to the MVVM Toolkit generators:

• Make Result<T> directly equatable
• Remove external comparers in IncrementalValue(s)Provider<T> and use equatable models directly
• Introduce a fully equatable DiagnosticInfo model that doesn't root any compilation item
• Fix diagnostics rooting symbols passed as formatting arguments
• Fix ValidationInfo not being properly equatable
• Remove unnecessary projections
• Port ObjectPool<T> type from Roslyn
• Add new ImmutableArrayBuilder<T> wrapper over ObjectPool<T>
• Use ImmutableArrayBuilder<T> everywhere to reuse ImmutableArray<T>.Builder instances

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]

Reverting to draft as I want to explore pooling the ImmutableArray<T>.Builder instances across generators as well, and it makes sense to batch that into this same PR since it's just about general source generators improvements and optimizations anyway 🙂