Add support for 'browser_specific_settings' as an alias to 'applications' in the manifest

[wagnerand]

Following mozilla/addons#6180, let's add support for browser_specific_settings to the linter as well.

I haven't checked in detail what needs to be updated, but there are a few code paths that touch applications. We should probably also have at least one test that uses browser_specific_settings.

Note: This should already be in the schema, if I understand correctly, added as part of https://bugzilla.mozilla.org/show_bug.cgi?id=1262005.

[eviljeff]

btw, browser_specific_settings seems to have been scrubbed from MDN now (apart from a reference on https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Browser_compatibility_for_manifest.json describing browser_specific_settings as a "non-standard" variant of applications).

[wagnerand]

@eviljeff yes, we reverted the changes on MDN until this has landed on prod.

[AlexandraMoga]

Verified fixed on AMO -dev with FF63, Win10x64

Extensions using browser_specific_settings in the manifest are treated equally as those using applications