[Fix comment in #445] Proper font lock in (s/def ::keyword)

[carlosgeos]

Fixes what is said here: #445 (comment). The regex could be improved so feedback appreciated. Basically, it has to exclude words starting with : so it does not apply function-name-face to them (they are keywords)

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[cichli]

Thanks for tackling this. All looks good to me, please squash the commits though.

[cichli]

Thanks again for this set of PRs, it's greatly appreciated! 🙌