[LiveHTML] Change heuristic for when to incrementally update

* Only incrementally update if the added/removed text doesn't contain
  any characters that could change the DOM structure.
* Fixed up some test expectations for edits that used to be incremental
  but aren't now (specifically, adding/deleting attributes).

@dangoor 

It's possible this is *too* conservative now--perhaps adding/deleting attributes should still be incremental (so we should consider `=`, `'`, and `"` safe). I guess I was concerned that because of the issues with unbalanced quotes, there might be some case where we get confused if we don't do a full tokenization of the file, but I don't have a specific use case in mind. Let me know what you think.