Avoid ubiquitous `tree.id!` null escape hatch

I've ported the source to use strictNullChecks, but a bunch of `!` operators appear on the `id` field of syntax nodes. (The TS calls these "null assertions," but that's misleading—they are unchecked, so they're not very assert-y.) We should replace these with a safe way of enforcing that the `id` field exists on stamped ASTs.