Bootstrap/Load: Introduce `wp_get_environment_type()` to retrieve the current environment type.

The type can be set via the `WP_ENVIRONMENT_TYPE` global system variable, a constant of the same name, or the `wp_get_environment_type` filter.

Possible values include `development`, `stage`, `production'. If not set, the type defaults to `production`.

Props Clorith, krogsgard, joostdevalk, frank-klein, Maelacuna, nathanrice, grierson, jchristopher, davidvee, jackfungi, johnbillion, tabrisrp, knutsp, ev3rywh3re, Rastaban, danielbachhuber, pfefferle, audrasjb, SergeyBiryukov.
Fixes #33161.
Built from https://develop.svn.wordpress.org/trunk@47919


git-svn-id: https://core.svn.wordpress.org/trunk@47693 1a063a9b-81f0-0310-95a4-ce76da25c4cd