Add commands to add or remove entries

[MalteHerrmann]

To add changelog entries that are correctly formatted, there should also be a simple way using CLI commands.

This should check e.g. if there is an open pull request for the checked out branch, that the branch is not main or whatever the base branch is, and ideally already prefill the PR title as the changelog entry.

[MalteHerrmann]

Update: For now there's no reason to add the remove subcommand for entries so it will be skipped.