Show list of environment variables in actions' parameters table

## Description

My proposal is to have actions' environment variables displaying in the their Parameters table, e.g. this one:

The environment variables would be a new column.

This would save users from being forced to run `fastlane action lane_name` when they need to know the env var for their action (which's probably the most common scenario nowadays, specially for CI implementations).

I know that these tables are automatically generated, but I have zero knowledge on how to implement this as of today 😄 I'd like to hear some feedback on this idea!