Terragrunt Prefix Env Vars

Throw an error when using the TERRAGRUNT_ prefix for environment variables.

terragrunt-prefix-env-vars - Reason

This prefix has been renamed to TG_ to shorten the prefix, due to the work in RFC #3445.

terragrunt-prefix-env-vars - Example

The TERRAGRUNT_LOG_LEVEL env var is deprecated and will be removed in a future version. Use TG_LOG_LEVEL=info instead.