nu: Copy basic sh aliases

Held back a little by the issue that you can't really set aliases
conditionally (yet?) in nushell, which is kind of fundamental to my
zsh/sh alias setup. If a program exists, we add some nice aliases -- if
not, we simply degrade gracefully and don't add anything.

PR to track issue:
https://github.com/nushell/nushell/issues/5068

terminal/.config/nushell/config.nu

@@ -146,17 +146,3 @@ $env.config.keybindings = [
         }
     }
 ]
-
-alias l = ls
-alias cl = clear
-
-alias md = mkdir
-def --env mcd [path: path] {
-    mkdir $path
-    cd $path
-}
-
-alias v = nvim
-
-# requires minimum nushell: 0.103 for job control
-alias fg = job unfreeze