Support for WP_Environment_Type

`WP_Environment_Type` is a new feature in 5.5. I would love to see postmark support it, so we can say have the default stream or server change based on the type. So Production, Staging, Dev, etc can be different.

https://make.wordpress.org/core/2020/07/24/new-wp_get_environment_type-function-in-wordpress-5-5/