(ruby-ts-mode): Split font-lock feature 'builtin' into two

(ruby-ts-mode): Split font-lock feature 'builtin' into two

* lisp/progmodes/ruby-ts-mode.el (ruby-ts-mode):
Split the 'builtin' feature in two: 'builtin-constant' and
'builtin-variable'.
(ruby-ts-highlight-predefined-constants):
Drop the user option, the highlighting granularity can be chosen
using treesit-font-lock-recompute-features.
(ruby-ts--font-lock-settings): Update accordingly.
(ruby-ts--predefined-constants, ruby-ts--predefined-variables):
Fix docstrings.