extensionBase: Add static lookupByURL() method

extensionBase: Add static lookupByURL() method

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>