Adding a few bindings to bring first class feeling multiselect to zed's
vim emulation.
gn and gN are similar to similar vim bindings, ga is similar to gA (and
I doubt we need vim's real ga), g> and g< are just made up.
Release Notes:
- vim: `g n` / `g N` to select next/previous
- vim: `g >` / `g <` to skip current selection and select next/previous
- vim: `g a` to select all