Draft: extensionSystem: Support ini metadata

JSON is a natural choice for a format that is shared between desktop JS (gnome-shell) and a web app (extensions.gnome.org), but it has a downside:

It is hard to integrate with our translation framework, so there is currently no way to localize user-visible strings like name and description.

To address this, support reading metadata from a keyfile instead, which has a well-established way of providing translations.

Fixes #2288