Fix non-termination in parser

[aibaars]

My earlier changes #238 somehow caused the parser to hang on some inputs. I'm not sure how it ended up in an infinite loop, but the changes I made to the scanner were not entirely right. I changed the scanner to return false if it sees a newline before a . character that has no following . character otherwise we'd end up calling mark_end, advancing a bit, and subsequently trying to scan a next token which makes no sense.

@hendrikvanantwerpen

Checklist:

• All tests pass in CI.
• There are sufficient tests for the new fix/feature.
• Grammar rules have not been renamed unless absolutely necessary.
• The conflicts section hasn't grown too much.
• The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c).

[dcreager]

Thanks for the quick debug on that @aibaars!

[hendrikvanantwerpen]

It's a bit tricky to follow the control flow here. I certainly didn't see that possibility last time either.

Afaict these changes do prevent the problematic case you describe, so approved!