wip(colors): expose highlight groups for user-defined priorities

[levouh]

Generally speaking, this PR serves to allow the user to do something like:

hi link OrgPriorityC @text.note

to get something like:

Officially it does not seem there is support for highlighting priorities, so I'm not sure if this should play the same as something like :h orgmode-org_todo_keyword_faces when defining the highlights.

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Acknowledging that there still need to be:

• [ ] Tests
• [ ] Documentation updates

but wanted to throw out a draft to see the approach that should be taken for this.

Wanted to get some feedback on:

• How the highlights should be exposed:
  1. Just add documentation for these highlights and allow the user to define them the "Neovim way".
  2. Add a new org_priority_keyword_faces option to allow the user to define them the "Org way". This would also be inline with the way that highlights work for TODO keywords, regardless of how ergonomic/normal it is for the Neovim user.
• Whether pulling the "highlights that come from user configuration" into a separate module in this manner makes sense.
• Other things as noted through self-comments on the PR.