This fixes a bug that could cause the cursor to incorrectly report its
start when using `slice` or `seek_forward`, and then calling `prev`. We
didn't notice this because we were not testing those three methods
together.
I suppose this could explain some of the panics we've observed because
we do use `slice`/`seek_forward` followed by `prev` calls in production.
This lets us use `next` or `prev` to decide whether to park the cursor
at the first or last filtered item.
Co-Authored-By: Nathan Sobo <nathan@zed.dev>
This required indexing UTF-16 positions in `Rope`. We tried opting
into the UTF-8 experimental support but it didn't seem to work
correctly and the standard is UTF-16 anyway.
Co-Authored-By: Nathan Sobo <nathan@zed.dev>
