Support alternate default branches for CI badge

[danielbachhuber]

CI badge generation is currently hardcoded to master:

https://github.com/wp-cli/scaffold-package-command/blob/f5684be0aef1538f5208486716c1a4fced44747d/src/ScaffoldPackageCommand.php#L258-L263

This repo now uses main. trunk is also common.

Done is:

• There's a flag to manually specify the preferred 'main' branch for the package.

[wojsmol]

@danielbachhuber Detecting for all setups wee be hard but we can add a parameter and set default value to for example main.

[danielbachhuber]

@wojsmol I'm fine with skipping the "default branch detection".