Export mantle environment in outputs

[Awes-3]

What type of feature request is this?

• Mantle doesn't support a Roblox feature I would like to use
• Mantle CLI improvement
• Other

Describe your problem
I would like to expose different features of my game depending on what environment the game was deployed from. An example would be letting a "QA Tester" role in a group access a debug menu in a development environment, but restricting access to that menu in production.

Describe the solution you'd like
Mantle could export the environment you are deploying from in the outputs JSON file.

[blake-mealey]

Good idea, I've updated #132 to include this