Add "unstable" functionality

[mansona]

release-plan was originally extracted from embroider to manage the release of a complicated monorepo tree of dependencies.

Embroider also has another feature, "unstable releases" that was not managed by release-plan because it pre-dates the creation of this package. The unstable releases always just do a next patch version and add -unstable.<sha> to the end but it would be nice to add this functionality to release-plan so it would actually be a major if the next release would be a major 👍

Here is the code for the unstable release: https://github.com/embroider-build/embroider/tree/main/test-packages/unstable-release

[RobbieTheWagner]

This sounds similar to what I need, I just need a way to specific prerelease for the GitHub release and an npm release channel, which could be alpha etc, as well as not just assuming a major version, but instead allowing overriding version numbers.