Expose some user preferences ($SHELL and $EDITOR) [FR: plugin API]

[Zykino]

The new #2862 sounds good, but so much peoples want to run a command in their shell, and I would love to be able to have an easy wrapper for them. (An easy $SHELL -c <cmd> <args> where they just give the cmd and args.)

I’m not sure if the $EDITOR is useful right now… but with it in the plugin I would like to (optionally) set it as environment so I would be able to fix #2582 at least when starting from the plugin.

For this preferences I don’t care if it come from the environment variable when starting zellij or from the config file. I just want the one zellij compute and would use when starting a terminal/edit pane.