feat(ci): add epic/* release workflow and rename master to trunk

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

1. Adds a new epic/* release workflow for creating release builds that we can use for beta testing and other QA purposes. This workflow is similar to alpha and hotfix/* workflows, but kind of in between them.
2. Renames the master branch to trunk.

Note: Once approved and merged, we should release a new version of this package to NPM and then update the dependency version number in Automattic/newspack-plugin#2895.

How to test the changes in this Pull Request:

Follow testing instructions in Automattic/newspack-plugin#2895.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[matticbot]

🎉 This PR is included in version 5.3.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀