endless heredocs don't consume rest of document

[nobodywasishere]

Closes #23

This works on paper but not 100% as I'm not as familiar with the heredoc parsing here.

Also, this change could go either way, as it's debatable which is preferable. I think this way is better as when adding a new heredoc to the middle of a document, you don't have to deal with everything else in the document being turned into a string until you add the heredoc end. But if you're changing the heredoc name, you may end up with the contents inside parsed as Crystal.

[keidax]

The implementation looks good.

I don't have a strong opinion about whether this change is an improvement. But I'm leaning towards not making this change, just because it's a little surprising.

My experience with Ruby syntax highlighting in the past has always been that a missing heredoc terminator causes the rest of the document to be highlighted as heredoc contents. tree-sitter-ruby also works this way.

[nobodywasishere]

That's valid. My thought process with this is just to limit the scope of errors as much as possible. I can leave this as a draft PR if we ever decide to come back to it.