Add `tar_config_list()`?

[psychelzh]

Prework

• [x] I understand and agree to help guide.
• [x] I understand and agree to contributing guide.
• [x] New features take time and effort to create, and they take even more effort to maintain. So if the purpose of the feature is to resolve a struggle you are encountering personally, please consider first posting a "trouble" or "other" issue so we can discuss your use case and search for existing solutions first.

Proposal

When there are multiple projects, listing all the projects is especially useful when choosing which project to use.

[wlandau]

Thanks, added tar_config_projects() to list projects and tar_config_yaml() to read the whole yaml file.