Migrate extension.json to manifest_version 2

Authored by Lucas_Werkmeister_WMDE on Fri, Apr 21, 10:24 AM.

Description

Migrate extension.json to manifest_version 2

This allows us to document the configuration options, which I believe
will be very useful once we start adding all the options for importing
constraints from property statements (lots of configurable property IDs
and item IDs).

Change-Id: Ic7ad9af6760c4eeac6640af72d9facc2d0b8de88