Support Vercel Custom Environments (variables)

[mountainash]

• 🔗 Custom Environments are now available on Vercel

Vercel now can have EnvVars for more than just Production & Preview.

The PRODUCTION input key for this project should change to be able to handle a name/string being passed that matches a Environment created on Vercel (Pro & Enterprise plans... so I guess we need to keep this for free accounts).

This is similar to how GITHUB_DEPLOYMENT_ENV key for this project already works. Maybe it already does???