Bug 41293: Register Torbutton as a l10n source in the jar.mn

With Firefox 102, we have switched from a JS implementation of Fluent to a Rust one, and the related API has slightly changed.

This commit uses packagedLocales already because we intend to switch to them, and at that point we will be able to use that variable to know which are the locales we actually support.

Part of the changes needed for tor-browser#41293 (closed).