Add support for `undefined` highlighting

[lienotly]

Hello there,

My team's using this parser as a part of @codemirror/lang-javascript with CodeMirror 6 for syntax highlighting.

However, we've noticed that the keyword undefined doesn't get highlighted, and while poking around, noticed that this parser's highlight.js doesn't seem to associate it with any tag.

I don't have much context, but from my understanding, if the change isn't much more than adding:

undefined: t.atom,

I'd be happy to open a PR!

[marijnh]

undefined is a regular variable in JavaScript, not a keyword, which I decided to reflect in the grammar.