Highlight type parameters in `declare` forms

[Izaakwltn]

Currently, when using lisp-mode, type parameter keywords are highlighted.

For instance in:

(declare copy ((Into :a Pathname) (Into :b Pathname) => :a -> :b -> (Result FileError Unit)))

:a and :b will be recognized as a different color.

[Izaakwltn]

Since Coalton doesn't use :keyword arguments for anything else, you could probably get away with just doing a blanket rule for colon-prefixed variables.