Feature/482 enum guard is defined

[szv]

Closes #482

PR Summary

Adds Guard.IsDefined to validate that an enum value corresponds to a declared member.

• Introduces a generic enum guard with caller argument expression support.
• Caches enum values in a FrozenSet to avoid repeated reflection and provide fast lookups.
• Adds a dedicated throw helper with a descriptive ArgumentException.
• Supports .NET Standard targets through the required collections package.
• Adds unit tests covering cache initialization, valid values, and undefined values.
• Exposes internals to the diagnostics test project for direct cache validation.

Alternative Approach

An alternative would be an enum source generator that detects enum types used with Guard.IsDefined and generates specialized membership checks, such as switch-based implementations. The generated code could avoid the runtime Enum.GetValues initialization, frozen-set allocation, hashing, and the internal use of Enum.IsDefined, potentially improving startup and lookup performance.

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
• New component Guard method
  • Pull Request has been submitted to the documentation repository instructions. Link: Add documentation for Guard.IsDefined enum validation MicrosoftDocs/CommunityToolkit#657
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that) -> to be honest, I don't know what you mean with that
• 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