Add small gnome-shell-extension-prefs script

Commit d76162c1 removed the ability to pass an extension UUID to the Extensions app, when we moved the dialog to a portal and made gnome-shell use it instead of spawning the extensions app.

However that missed that many extensions called out to the app to open their own prefs.

While extensions are encouraged to switch to the new openPrefs() convenience method added in commit 8030d9ad, restore the old behavior with a small script under the old gnome-shell-extension-prefs name that either calls out to the portal or launches the app.