vim: Display pending keys in Vim mode indicator (#13195)

This changes the mode indicator to now show pending keys and not just
pending operators.


Release Notes:

- Added pending keys to the mode indicator in Vim mode.

Demo:



https://github.com/zed-industries/zed/assets/1185253/4fc4ffd9-2ba7-4e2c-b2c3-cd19b40cb640