Llvm-diagnostics

Removed
- Renamed internal classes (removed superfluous `Diagnostics`-prefix)

Added
- Ability to configure the formatting using a global configuration method (`llvm_diagnostics.config(...)`)

Added
- Documentation (incl. Contribution, Security, Maintenance)

Changed
- Indicators are now displayed in red in case there is no expectation given

Added
- Add support for Python versions `>=3.7`

Fixed
- Diagnostics messages without a line are now properly reported with the GitHub formatter

Removed
- Removed the `to_json()` method as it was only used for validation

Added
- New classes representing the message type (`DiagnosticsError`, `DiagnosticsWarning`, `DiagnosticsInfo`)

Changed
- Allow creation of a diagnostics message without `file_path`