Add customizable Themes and Highlighters for rich colors in print_msg

[Crivella]

This is a followup on

• Add rich colors to print_msg #5125
• make EasyBuild plugin-able through entrypoints #4918

With this PR, Easybuild ships its own highlighter and theme that can be replaced using entrypoints (for now only the theme) using the following groups

• easybuild.rich_theme: A dictionary that defines the rich theme to be used

TODO:

• Expand what EB should highlight by default
• Define a good color scheme to avoid too much clutter
• Add to the entrypoint documentation
• Tests

Example of themes via entrypoints:

• Crivella/test_eb_ep@5ea8a58

[Crivella]

@boegel @smoors Let me know design-wise what you think about this, if it is worth investing more time in trying to expand this more (eg also make the highlighter pluginable).

In general going to need feedback on colors and patterns, my UI design skills are not the best so i am afraid of going full rainbow with the colors