'treesit-outline-search' didn't match outlines at the beginning of
the buffer because unlike 're-search-forward' (used by outline-mode)
that matches the text that immediately follows point,
'treesit-navigate-thing' misses text at point and stars the search
after point.

So there is a need to handle this difference specially.  Therefore this
patch adds such special-handling of bobp to 'treesit-outline-search':