Bug 1836585 - Change the "Top pick" label to "Firefox extension" for addon...

Bug 1836585 - Change the "Top pick" label to "Firefox extension" for addon suggestions. r=daisuke,fluent-reviewers,flod, a=dmeehan, l10n=flod

This uses the new label but only when the addon suggestion is shown as a best
match. Otherwise the suggestion will be shown in the usual Firefox Suggest group
so it should use that label.

I chose `-brand-product-name`, which is always "Firefox" even on Nightly. I
don't think it makes sense to talk about "Nightly extensions".

Differential Revision: https://phabricator.services.mozilla.com/D179868