Requests for comment/Extension registration/Schema

Most of the keys are either from or the name of the global variable with the "wg" prefix removed. This was up to date when originally created, but is not up to date with the current DefaultSettings.php.

Exceptions:
 * config: The configuration settings for this extension.
 * callback: A callback function to be run after this extension.json file is processed for setup that must occur in PHP.
 * namespaces: A combination of all the various namespace global settings.