Add a command to add a new package to a release

Sometimes you want to add a new package to a release after you create a release.

We could add a new command to `create-release-branch`, perhaps `add-package`, that would allow you to:

- Bump the version of a package (specifying `major`, `minor`, `patch`, or a version)
- Create a new section in the changelog for the new version
- (Optionally) Populate the "Uncategorized" subsection with changelog entries

This would be an alternate to https://github.com/MetaMask/create-release-branch/issues/28 (which is more about being able to run `create-release-branch` and edit the release spec or open the UI view again).

No files or tests are identified in the issue. Start by reviewing the create-release-branch CLI entry point and compare the proposed command with issue #28. Done should include a command that bumps a package version, creates its changelog section, and optionally populates "Uncategorized" entries.

Written by the indexing model from the issue text.