Bootstrap/Load: Make some adjustments to `wp_get_environment_type()`:

* Rename the `wp_approved_environment_types` filter to `wp_environment_types`.
* Introduce `WP_ENVIRONMENT_TYPES` system variable and constant to complement the filter.
* Correct the argument type for the `wp_environment_types` filter.
* Cache the result in a static variable to ensure consistent return value.
* Rename the `stage` type to `staging`.

Follow-up to [47919].

Props dlh, dd32, TimothyBlynJacobs, johnbillion, pbiron.
See #33161.
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git-svn-id: https://core.svn.wordpress.org/trunk@47957 1a063a9b-81f0-0310-95a4-ce76da25c4cd