Union return types for method must return type rule (#9)

* Improving the MethodMustReturnType rule

* Fixing edge cases

* Add regex support for method return type validation

- Introduced new classes for testing regex patterns in method return types.
- Enhanced the MethodMustReturnTypeRule to support regex matching for expected return types.
- Updated documentation to reflect regex usage in return type rules.
- Added comprehensive tests for various valid and invalid cases involving regex patterns.

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Co-authored-by: Florian Krämer <[email protected]>

Author: floriankraemer