This was originally just to fix
https://github.com/zed-industries/zed/issues/4354, which I did by just
returning the previous offset in `find_boundary`.. but `find_boundary`
is used in the "insert mode" / normal editor too, so returning the
previous boundary breaks existing functionality in that case.

I was considering a new `find_boundary` function just for some of the
vim motions like this, but I thought that this is straightforward enough
and future Vim functions might need similar logic too.

Release Notes:

- Fixed https://github.com/zed-industries/zed/issues/4354