More settings for github mode

[Matthijz98]

Just set up the Github mode for Keystatic for a site. But it would be nice to be able to set the following settings:

• Default branch. Because i want it not to be main but even when using the prefix setting the default is the main branch
• Settings some default settings for opening a pull request. I would love to set the default branch where to merge to. This could be done by adding some settings to the url for example when i want to merge content into main: /compare/main...Matthijz98:{REPONAME}:content?expand=1.

[SahilMahadwar]

+1