Implement a prompt section for Vi mode

[matchai]

Feature Request

Is your feature request related to a problem? Please describe.
This feature is intended to achieve feature parity with the following Spaceship section file: https://github.com/denysdovhan/spaceship-prompt/blob/master/sections/vi_mode.zsh

Describe the solution you'd like
Porting over the existing zsh implementation of the Vi mode section file, indicating whether or not the shell is set to Vi mode, which is enabled with fish_vi_key_bindings.

Useful discussion on fish vi bindings can be found here: fish-shell/fish-shell#3232

[kulabun]

I guess we can close this ?

[matchai]

Oh! Yes we can.
Thank you. 👍