Treat html pre and nowiki as extension tags
- Cleans out a lot of cruft from the tokenizer.
- The redundancy in the html pre extSrc is rather unfortunate, but at least it enforces text editing which is the right model.
- Support for pre tags in indent-pre, which was previously broken and disabled in T108216.
- No longer emitting self-closed nowikis as placeholder text. Instead, they're just empty spans as requested in T58381 for VE to drop.