Support starship integration with fish vi_mode variables

[varon]

https://starship.rs/config/#character

Note that we can configure this shell prompt by setting the various vi-mode symbols. This is very helpful when working in vi-mode in fish.

It would be great if these were set too.

[sshilovsky]

Currently $fish_bind_mode is set to default, insert, or visual. What variables does starship expect?