Tweak c-ts-mode fontification

Tweak c-ts-mode fontification

New features: function and variable, which consistently fontify all
occurrences of functions and variables, respectively.

Remove expression feature, as its purpose is fulfilled by function,
variable, and property combined.

Fix declaration feature, remove unnecessary rule from label
feature.

* lisp/progmodes/c-ts-mode.el (c-ts-mode--font-lock-settings): See
above description.
(c-ts-mode--fontify-variable): New function.
(c-ts-mode--base-mode): Add & remove features.