This PR makes two changes:

1. It fixes how the parser acquires the name of a PrivateIdentifier from the scanner, preferring the "cooked" tokenValue after processing escape sequences rather than just reading the plain source (the "token text"). Previously, two private names with the same effective/cooked names would not resolve to each other unless their source text was verbatim identical.
2. It fixes the scanner, which previously didn't expect a that a hash/pound (#) could be followed by a backlash (\).

There is still a slight problem around the fact that under ES3 and ES5 (our defaults), we don't recognize extended escape sequences at all. Ideally we would improve this by gracefully parsing and issuing an error that we can't emit them or something. But what that means is that I'm not yet certain as to whether the language service issue discovered within test262 (under formatjs) in #50835 is actually fixed by this. That might need to be a follow-up.

Fixes #50851.