Forward fish_key_bindings for fish 4.3 changes

[mkmkme]

This patch is forward-ported from IlanCosman#619 and fixes an inverted prompt sign

Personal note: thanks for keeping the project alive and maintained! There's one annoying issue in tide that still bothers me, and I would like to contribute the existing fix from the upstream repo to this fork.

Description

In fish 4.3.0:

fish no longer sets universal variables by default, which simplifies configuration. Specifically, the fish_color_, fish_pager_color_ and fish_key_bindings variables are now set in the global scope by default. After upgrading to 4.3.0, fish will (once and never again) migrate these universals to globals set at startup in the ~/.config/fish/conf.d/fish_frozen_theme.fish and ~/.config/fish/conf.d/fish_frozen_key_bindings.fish files. We suggest that you delete those files and set your theme in ~/.config/fish/config.fish.

This change forwards the fish_keybindings for the prompt as well as does a check if the fish_keybindings is not set.

Motivation and Context

Closes #

Screenshots (if appropriate)

How Has This Been Tested

• I have tested using Linux.
• I have tested using MacOS.

Checklist

• I am ready to update the wiki accordingly.
• I have updated the tests accordingly.

[plttn]

Thanks for the PR, I'll take a look at this but I don't see any reason why it shouldn't make it in.