Multi-trunk support

While most repos have a single trunk (e.g. main or master),
some projects operate in a multi-trunk mode, e.g.
a dev branch for ongoing development and a release branch for most recent release.

The feature request here is to support multi-trunk repos.
This would be a significant change to the internals/state management.
Off the top of my head:

- State currently only has a single trunk field; we'll need to support either `trunks: []string`, or `secondaryTrunks: []string`, while requiring one primary trunk. (This requires some thought — if there's any use case that requires a "primary" trunk, we'll want to do the latter.)
- PRs would be made against the trunk they're stacked on top of.