Escape non-ASCII identifier characters with `write!(…, "u{:04x}", …)`,
surrounding with `_` as appropriate. This solves (1) a debugging issue
where stripped characters would otherwise be invisible, and (2) failure
to re-validate that stripped identifiers didn't start with an ASCII
digit.
I've confirmed that this fixes [bug
1978197](https://bugzilla.mozilla.org/show_bug.cgi?id=1978197) on the
Firefox side.