[suggestion] Add syntax group for built-in module attributes

Would you be open to a PR that adds a syntax group for Elixir's built-in module attributes?

This is a nice-to-have which:

1) gives immediate feedback that a built-in has been spelled correctly (and indeed exists)
2) gives immediate feedback when trying to define a built-in as a custom attribute
3) gives a visual difference between built-ins and user-defined "constants"

Thanks for reviewing and no worries if the answer is no!