[LiveHTML] Always parse strictly, even for full parse

For #4734. @dangoor 

Originally, when updating after edits, we would do a strict parse for incremental reparses, but would do a lax parse for full reparses. I now think that's incorrect: we should basically refuse to do any updating while the document is incorrect. The *initial* parse when loading a file can still be lax, so that basic highlighting will work as well as it can, but I don't think we should try to do any live HTML updating while the document is invalid, because our edits are pretty much guaranteed to be wrong.

I definitely think we should couple this with some UI that indicates that updating isn't happening because the doc is invalid. Let's talk about that for next sprint.