cd99fbae50
With convenience API like getSettings() now being provided by
the ExtensionObject subclass, extensions will need to access
their entry point more often.
Having to pass a pointer through the hierarchy can be annoying,
so add a static method that allows them to look it up:
```js
const ext = Extension.lookupByURL(import.meta.url);
this._settings = ext.getSettings();
```
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2838>