vim: Add multicursor shortcuts (#3032)

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