Add two rows in `$nu`, `$nu.is-interactive` and `$nu.is-login`, which
are true when nu is run in interactive and login mode respectively.
The `-i` flag now behaves a bit more like that of bash's, where the any
provided command or file is run without REPL but in "interactive mode".
This should entail sourcing interactive-mode config files, but since we
are planning on overhauling the config system soon, I'm holding off on
that. For now, all `-i` does is set `$nu.is-interactive` to be true.
About testing, I can't seem to find where cli-args get tested, so I
haven't written any new tests for this. Also I don't think there are any
docs that need updating. However if I'm wrong please tell me.
7e949595bd