Clone current environment variables

[musjj]

Having the ability to clone the environment variables from the current shell session into a new pane can be really useful.

For example, direnv can set environment variables that modifies program's behavior, install new programs into the PATH, etc. Currently, when you run zellij run -- $ARGS, $ARGS won't be affected by the current direnv session. This will cause issues like missing programs, incorrect behavior, etc.

In kitty, you can do this with --copy-env. A similar option would be nice.

[cgbarthel]

tmux has this ability as well. In the config you can specify which environment variables (DISPLAY, SSH_AGENT, etc) you want to bring into a new tmux environment.

[dsully]

See also #1531

[lakshya-sky]

Related #2844 and #2639