feat(config): Add environment variables in layout pane commands

[glcraft]

Add the ability to set environment variables into pane commands from the layout configuration.

A env_vars property is added into pane node to define environment variables

Example:

layout {
    pane {
      command "zsh"
      args "-c" "echo ==> $TEST_FOR_ZELLIJ <=="
      env_vars {
        TEST_FOR_ZELLIJ "hello world"
      }
    }
}

Note : With this implementation, env_vars has to be a node with subnode (so not in kdl arguments form).

Note 2: This is my first PR, I hope I didn't miss anything aside from the implementation.

[glcraft]

Resolves #2988

[glcraft]

It misses the restoration after quitting a session and resurrect it. I have to fix that before merging

[glcraft]

It should be good now 👍