FIX: use `VISUAL=EDITOR` and not `VISAL=...` in the shells

[amtoine]

This PR

• fixes the name of the VISUAL environment variable in both nushell and bash
• sets VISUAL to the EDITOR

now, one only needs to change export EDITOR=... in the bashrc and let-env EDITOR = ... in config.nu to change the editor system wise :+1: