✨ add already existing directories

[jeftadlvw]

The add subcommand should support already existing subdirectories that are git repositories.

This should not be the default way of adding nested modules but rather be hidden behind a cli flag. In which case the default arguments are ignored.

git nest add --existing path/to/module

This firstly gets all required information from the module and if it's even a git repository. Then validation is performed to prevent duplicated, after which the module is added to the configuration.