[Feature Request] Support select pane after starting new session

## Overview Hi, I've just found this awesome project, and it fits my workflow very well It would be great if a specific pane can be selected by default after starting a new session ## Detailed Description Below is an example of my session configuration ```yaml # .laio.yaml name: dotfiles windows: - name: code panes: - flex: 5 # main editor commands: - command: nvim . - flex: 3 # terminal ``` It's a bit inconvenience because the terminal pane is always selected first, therefore I have to switch to the editor pane manually It would be great if we add a `focus` on the pane config, so the editor pane will be automatically selected after starting session ```yaml panes: - flex: 5 focus: true - flex: 3 ``` This is just the rough idea, we may have more work to do with window, etc... ## Benefits A specific pane can be selected by default without manual operations ## Alternatives I'm currently using tmux hook to switch to editor pane (usually pane 0), but it fails if the editor pane is not 0 ``` set-hook -g client-attached 'run-shell "tmux select-pane -t 0"' ``` ## Additional Context Add any other context about the feature request here.