Add MA0204 to remove unnecessary partial modifiers

[meziantou]

Summary

• Add MA0204 analyzer and code fix to report and remove partial when a type has a single declaration.
• Cover classes, structs, interfaces, and records, while preserving trivia during the fix.
• Update rule documentation, rule identifiers, and analyzer pack severity presets.
• Add unit tests for compliant and non-compliant cases, including nested partial types and partial members.

Testing

• Not run (not requested)