Add support for incremental parsing.

Right now the entrypoint just causes a full parse to happen.  But the LS code is
cleaned up to take advantage of it appropriately.

This will not be checked in until all the incremental parsing work is complete.
The implementation of `containsParseError` (in utilities) uses a `hasFlag` method instead of just checking the flag inline. I really don't like that and we never do that elsewhere.