fix(tokenizer): require full `-->` to close comments in xmlMode
Merged
FB
7mo ago+80+80+01234567890123456789-2-2-0123456789
In HTML, <!--> is a valid empty comment per the spec. However, in XML comments must be closed by -->, so <!--> should be treated as an opening — not a complete comment. Skip the short-comment shortcut when xmlMode is enabled.
Fixes #1823