We don't need to take care about SpellCheck right now as it doesn't use any `highlighting_is_*_style()` functionality yet, i.e. I need to touch the SpellCheck anyway in this regard.
Thanks for the great list of uses of `highlighting_is_code_style()` which is convincing to add regex styles to `highlighting_is_string_style()`.
Will do if @b4n and @elextr will agree.