extensionUtils: Add utility for setting the current extension

ES modules do not allow exports to be overriden, in anticipation
of the ESM port add a `setCurrentExtension` utility which will
throw if used in the shell. This is tested using a conditional
import of Main.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2363>

js/dbusServices/extensions/extensionPrefsDialog.js

@@ -18,7 +18,7 @@ var ExtensionPrefsDialog = GObject.registerClass({
             ExtensionUtils.installImporter(extension);
 
             // give extension prefs access to their own extension object
-            ExtensionUtils.getCurrentExtension = () => extension;
+            ExtensionUtils.setCurrentExtension(extension);
 
             const prefsModule = extension.imports.prefs;
             prefsModule.init(extension.metadata);