[selectors] Rewrote grammar in terms of the Value Definition Syntax

As I've been threatening to do for quite some time, I finally rewrote
the grammar for Selectors in terms of the CSS Value Definition Syntax,
removing the old grammar written in terms of the Appendix G syntax.
This is one more step in the effort to remove *all* grammars written
against the old Appendix G syntax from CSS, now that CSS is no longer
defined in a way that's directly compatible with it.

Once auto-generation finishes in a few minutes, you can find it at
<https://drafts.csswg.org/selectors/#grammar>.  Please review and let
me know if you notice any mistakes!

~TJ