Expand on list indexing syntax

[annevk]

I would like us to support list[last] or list[-1] at least. There's a lot of specifications that could benefit from that (instead they talk about the last item today, which works but is verbose and inconsistent with how we talk about the first item).

E.g.,

• https://url.spec.whatwg.org/#ends-in-a-number-checker
• https://url.spec.whatwg.org/#concept-ipv4-parser

Open to additional suggestions if we want to tackle a couple in one go. Though only as long as there is specification text that could benefit from them.