bug#80837: treesit-forward-comment off-by-one issue: overshoots by one character

>> -(defun treesit-forward-comment (&optional count)
>> +(defun treesit-forward-comment (count)
>
> I'd refrain from including this part of the change.

I don't remember why I made this argument optional.
Probably by mistake because it was intended to have
the same signature as `forward-comment` where the
argument is not optional.  So it would be nice to fix
this blunder before this function's first release.

> Also, I'm not sure we need the whole `treesit--likely-line-comment-p`:
> can we just test if `comment-end` is nil instead?

Testing for an empty `comment-end` should work at least
for c-ts-more and lua-ts-mode where the comment thing is set.