Add 'create' command to create empty Unity project

[chrisyarbrough]

The simple version of this command create a default Unity project via the Unity editor CLI (which has no options).

unity create path/to/project <version>

Next, we can make the version optional and add interactive selection, suggesting the newest one.

Then, we can add a create --minimal option which bypasses Unity's CLI and instead just created the necessary directories and ProjectVersion.txt file. Ideally, this also provides an empty manifest without any packages, so that it's quick to open this project for speedy experiments.