Add handling for double underscores

[tirix]

This is to align with change done in metamath/metamath-exe#160, to handle cases discovered in metamath/set.mm#3389.

Before these changes, double underscores were treated as subscript. They are now correctly ignored and returned as a single span.

To be complete, in addition to these changes, the unescape_text function in CommentItem should also transform double underscores into single underscores. @digama0, what's your proposal for those?

[digama0]

@tirix I made a modification here to inline get_underscore_mode since the advantage of having a separate function is reduced if it is not using ? anymore, but maybe the new code is a bit too terse. LMK what you think.

[tirix]

@tirix I made a modification here to inline get_underscore_mode since the advantage of having a separate function is reduced if it is not using ? anymore, but maybe the new code is a bit too terse. LMK what you think.

Yes indeed, and there is no need for a special enum for that. Ok with the change.