Fix completion on first character

[tboby]

This fixes completion on the first character (or no characters) of a new expression. This substantially improves the suggestions workflow in VS Code.

This is done by adding another lookup kind for the Lexer which doesn't allow too many tokens from the left to get picked up. This prevents an operator or whitespace being picked as the current token which breaks later parts of the completion logic.

This is still a workaround, as I still don't fully understand the whole TryGetCompletions process. It doesn't fix completion everywhere either. For example, after the < of a type param. However, < isn't a trigger character so it isn't as much of a problem.

[baronfel]

This is great! Thank you for adding tests. I'm happy to see that you found a less-impactful change than copying the entire longIdent-finding function like we were talking about :)

I'm very happy to take this change pending green tests, and we can look into any more comprehensive work building off of your tests.