[ticket/10824] Move style configuration from style.cfg to composer.json

phpBB itself uses composer.json, as do extensions, so for consistency, styles are being changed to do so as well (eventually language packs will too, but that is beyond this ticket). This ensures that each component uses the same file and format.

This also allows us to eventually allow installation of styles directly from the CDB through the ACP (as is planned for extensions and potentially language packs as well).