extensionUtils: Split out extension convenience functions

ExtensionUtils was originally used for shared functions between
the extension system and the (old) prefs-tool, but then gained
useful API meant for extensions themselves.

It's a bit weird to mix the two, so split out the extension convenience
API into a separate module.

We will soon split up the module further, and add specific "flavors"
for extensions and preferences, with the current code providing a
shared base for both.

That should explain both the new location and the odd module name. :-)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2837>

js/dbusServices/extensions/extensionsService.js

@@ -7,7 +7,7 @@ import {ExtensionPrefsDialog} from './extensionPrefsDialog.js';
 import {ServiceImplementation} from './dbusService.js';
 
 const {deserializeExtension} = imports.misc.extensionUtils;
-const {setExtensionManager} = imports.misc.extensionUtils;
+const {setExtensionManager} = imports.extensions.sharedInternals;
 const {loadInterfaceXML} = imports.misc.dbusUtils;
 
 const ExtensionsIface = loadInterfaceXML('org.gnome.Shell.Extensions');