Allow background.type for MV2

[mischnic]

No need to be overly restrictive here. Though we don't actually use that value in the transformer right now

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/background

[101arrowz]

If we're going to support module-type background scripts, we should probably manually handle background.scripts like we do for background.service_worker, rather than leaving it in DEP_LOCS. We'd need to set the env.sourceType to the type property to program.background.type == 'module' ? 'module' : 'script', just like we have for service worker extensions.