environment: Port environment.js to be an ES module

Removes the init() function in favor of executing all environment
changes when the file is imported.

Additionally ports all unit tests using imports.gi.environment.init() to
use the updated module.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2822>

.eslintrc.yml

@@ -4,6 +4,7 @@ extends:
 overrides:
   - files:
       - js/ui/init.js
+      - js/ui/environment.js
       - js/dbusServices/**
       - js/portalHelper/**
       - subprojects/extensions-app/**