MAINT: Spinning out the `.el` font-locking

[HaoZeke]

As noted here https://github.com/melpa/melpa/pull/9380#issuecomment-2691707940

It would be nicer perhaps to have the font-locking live in a separate repository. If not, a PR to the current repo could also be another option.

[kentonv]

Hi @HaoZeke,

If you want to send PRs here, my teammate @justin-mp -- who is a daily user of emacs and capnp-mode -- can review and merge. If you'd rather make a fork that's entirely up to you of course.