Environment: Add to `new-window`, `split-window`, `respawn-window`, `respawn-pane`

Works as of tmux 3.0 ([changes](https://github.com/tmux/tmux/blob/3.3/CHANGES#L816-L817)):

> * Add a -e flag to new-window, split-window, respawn-window, respawn-pane to
> pass environment variables into the newly created process.

# Examples

```
tmux split-window -e MOO=3 -e BARK=5
```