Danger-swift should default to `main` and not `master`

It's been almost four years since [Github decided to rename master to main](https://github.blog/changelog/2020-10-01-the-default-branch-for-newly-created-repositories-is-now-main/), and we still have to specify `--base main` in Danger lest we get an error.

I'm not talking about renaming this repository's master branch, that's up to you. But having a sensible default when using Danger on other repos seems like a logical thing to do.

Start by tracing how Danger Swift handles the --base option and where its default branch is selected. Review any tests covering base-branch selection; done means using Danger on a repository with a main branch works without specifying --base main, while existing behavior remains covered.

Written by the indexing model from the issue text.