Fix highlighting of callable keyword

[fabacino]

If the keyword callable is used as a type hint or a return type, it is highlighted as a keyword instead of a type.

Example:

public function decorate(callable $function): callable
//                       ^^^^^^^^             ^^^^^^^^

This commit fixes this inconsistency and highlights it as a type in these situations.

[zonuexe]

Before

After

[zonuexe]

@fabacino Thanks a lot!