Refactor DiagnosticsFormatter and extend its functionality

This commit refactors the existing code for `DiagnosticsFormatter` and introduces several new features, complete with documentation and unit tests.

Key Enhancements:

1. Nested Diagnostic Support: Enhanced to include not only top-level diagnostics but also related notes, improving the debugging experience.

2. Custom Decorators: Incorporate the `DiagnosticDecorator` protocol, allowing for custom formatting and styling of diagnostic output.

3. Context Size Control: Added options to control the `ContextSize`, providing more flexibility in how much source code context is displayed around each diagnostic.

Documentation:

- Comprehensive documentation added, detailing the purpose, usage examples, and future developments for `DiagnosticsFormatter`.

Testing:

- Added robust unit tests to validate the new features and ensure reliability.

This refactor and feature addition make `DiagnosticsFormatter` a more versatile and developer-friendly tool for debugging and understanding Swift code.