Support VIM style keybindings with custom leader key in VIM normal mode

[hajdamak]

VIM normal model keys are already used for many actions. Setting new VIM style bindings is limited.
Nushell should support defining keybindings with leader key like in VIM, Fish etc.
So you can choose your leader key (for example Spacebar) and then in VIM normal mode define keybindings like g or f that do not collide with regular VIM mode bindings.