Add option to configure the default focused startup panel

### 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 https://github.com/jesseduffield/lazygit/discussions/3189

Start by tracing lazygit startup and the existing config-file options, then compare the default launch with invoking lazygit using `log`. Add a configurable startup panel choice and verify that a fresh launch focuses the selected panel without requiring an argument.

Written by the indexing model from the issue text.