Improve support of extend-protocol forms

[rrudakov]

• Highlight function name with font-lock-function-name face
• Support of clojure-ts-add-arity command

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• The commits are consistent with our contribution guidelines.
• You've added tests (if possible) to cover your change(s). Bugfix, indentation, and font-lock tests are extremely important!
• You've run M-x checkdoc and fixed any warnings in the code you've written.
• You've updated the changelog (if adding/changing user-visible functionality).
• You've updated the readme (if adding/changing user-visible functionality).

Thanks!

[rrudakov]

I think if we just restart the CI, it should be OK:

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