Feature Request: Add generators

[axelson]

We were going to add generators in https://github.com/ScenicFramework/scenic_new/pull/53

But these make more sense to include in Scenic itself since the version of scenic_new may not match the version of scenic

What problem does this feature solve?

Makes it easier for developers to create new resources within Scenic such as new scenes

What is your proposed solution?

A mix scenic.gen.scene command that will generate a new scene (and possibly other generators as well)