@ForLoveOfCats and I found a few speedups that make this acceptably fast
(able to update ~10k selections in <100ms), so the remaining work here
is to fix the tests, and then ship!

Release notes:
- Updated up/down to work based on pixel positions
([#1966](https://github.com/zed-industries/community/issues/1966))
([#759](https://github.com/zed-industries/community/issues/759))
- vim: Fixed off-by-one in visual block mode
([2123](https://github.com/zed-industries/community/issues/2123))