Set WP_ENVIRONMENT_TYPE for known staging sites

[joemaller]

Collect staging identifiers from Kinsta and WP-Engine, then use those to set the WP_ENVIRONMENT_TYPE to staging.

Kinsta appears to be set $_SERVER['WP_ENVIRONMENT_TYPE'] to staging for staging environments.

More info from the Version Info, but I'm not seeing any of these in Site Health -> Info or $_ENV vars via phpinfo().

Detection Sources

Version Info automatically detects the environment from multiple sources, checking them in order of priority:

1. WP_ENVIRONMENT_TYPE - WordPress core constant (since WP 5.5)
2. WP_ENV - Used by Bedrock and Trellis setups
3. Kinsta - KINSTA_ENV_TYPE constant
4. WP Engine - WPE_ENVIRONMENT and IS_WPE_SNAPSHOT constants
5. Pantheon - PANTHEON_ENVIRONMENT constant
6. Flywheel - FLYWHEEL_CONFIG_DIR constant detection