ruby-ts-mode: Highlight builtin methods

ruby-ts-mode: Highlight builtin methods

* lisp/progmodes/ruby-mode.el (ruby-builtin-methods-with-reqs)
(ruby-builtin-methods-no-reqs): New constants, extracted.
(ruby-font-lock-keywords): Replace values with references.

* lisp/progmodes/ruby-ts-mode.el (ruby-ts--builtin-methods): New
variable.  Construct regexp from aforementioned constants' values.

* lisp/progmodes/ruby-ts-mode.el (ruby-ts--font-lock-settings):
Use it.

* lisp/progmodes/ruby-ts-mode.el (ruby-ts-mode):
Add new font-lock feature: builtin-functions.

* lisp/progmodes/ruby-ts-mode.el (ruby-ts--predefined-constants)
(ruby-ts--predefined-variables): Unrelated to the rest of the
patch, add string-start and string-end anchors.