Ability to set prerelease version suffix separately from branch name

**Is your feature request related to a problem? Please describe.**

For a project which publishes commits to its `main` branch as prerelease versions, and would like to use a version matching the pattern `1.0.0-beta.1` or `1.0.0-next.1`), it would be nice to configure this. Currently, auto will use the the pattern `1.0.0-main.1`.

**Describe the solution you'd like**

The `prereleaseBranches` configuration could be updated to adopt a more detailed configuration, like:

```json
{
"prereleaseBranches": [{"branch": "main", "label": "beta"}]
}
```

**Describe alternatives you've considered**

One alternate solution is to rename the branch from `main` to `beta`, but that's less clear. In git, "main" (or "master") is typically the branch pull requests get merged to.

A second is to keep `1.0.0-main.1` as the prerelease version for releases produced from the `main` branch, but that is less clear from a versioning standpoint. "beta" has an established meaning. And for flexibility sake, it would be nice to decouple the two concepts.

A third is to abandon the concept of preproduction releases and give every commit a full production release version. This isn't appropriate for libraries that do produce preproduction versions for testing before blessing something as a full release.