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>

tests/unit/signalTracker.js

@@ -2,16 +2,14 @@
 
 // Test cases for version comparison
 
+import '../../js/ui/environment.js';
 import GObject from 'gi://GObject';
 
 const JsUnit = imports.jsUnit;
 const Signals = imports.misc.signals;
 
-const Environment = imports.ui.environment;
 const {TransientSignalHolder, registerDestroyableType} = imports.misc.signalTracker;
 
-Environment.init();
-
 const Destroyable = GObject.registerClass({
     Signals: {'destroy': {}},
 }, class Destroyable extends GObject.Object {});