Add option to configure the default focused startup panel #5877

Is your feature request related to a problem? Please describe.

I'd like to be able to configure which side panel is focused on lazygit startup, without having to pass any additional arguments to lazygit like lazygit log.

I'm used to tig's startup approach, which would be focusing the Commits panel, rather than the Files panel.

Describe the solution you'd like

Add an option to the config file to select the startup panel.

Describe alternatives you've considered

Invoking lazygit with log is an alternative, but it's more to type, and having aliases based on shell or editor, is more over overhead.

Additional context

This was previously mentioned in the discussion at #3189