This commit creates 4 new highlight groups:
- LspDiagnosticsErrorSign
- LspDiagnosticsWarningSign
- LspDiagnosticsInformationSign
- LspDiagnosticsHintSign
These highlight groups are linked to their corresponding LspDiagnostics
highlight groups by default.
This lets users choose a different color for their sign columns and
virtualtext diagnostics.