Write a couple of unit tests

[FourierTransformer]

I think having unit tests (or coverage) for at least the following would be good to start with:

• Document:get_tree_sitter_token
• Document:type_information_for_tokens

It could potentially be extended to the completion method in general, but the above two really power it and are sorta the core logic in the teal-language-server.

I'd be a little cautious of testing the formatters, since they are the most likely to change (and are just for display purposes). Having tests for the document handling, and message passing would also be good.