Use `rich.highlighter.RegexHighlighter` for help screen colouring?

[kdeldycke]

This might simplify the management of regexes in the help screen highlight code.

See: https://github.com/Textualize/rich-cli/blob/0721cb196661687d8ecd9af4baf48576b240c978/src/rich_cli/__main__.py#L175-L179