Provide preset environment variable to distinguish Preview vs Production Deployments

It would be useful for code to be able to determine which type of deployment it is running within via environment variable. Right now I have to manage this with a flag in code that only lives in the main branch, and be careful not to invalidate that flag during merges.

I can give more details on my specific use case if it is of interest.

Start by reading the issue and its three comments to understand the requested distinction between Preview and Production Deployments and the missing deployment details. Done means the platform provides a documented preset environment variable that reliably identifies the deployment type.

Written by the indexing model from the issue text.