Support `WP_ENVIRONMENT_TYPE`

[LeoColomb]

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Since version 5.5, WordPress supports WP_ENVIRONMENT_TYPE environment variable and wp_get_environment_type function.

Using this constant/function could be really useful to avoid publishing content to an attached Discourse instance while not in production. For example, JetPack do not send any content to wp.com if environment is staging or development. Doing the same with wp-discourse would help managing staging environments.