Add a temporary project to fetch Fluent tranlations for base-browser

For the language notification box (tor-browser#41378 (closed)), I have chosen to use Fluent for strings.

It's the first component to use this format, because the Weblate support for it isn't mature yet and cannot deal with the other strings we have.

However, these strings are very simple, so it made more sense to start directly with Fluent for me, instead of creating them in the other format and then ask users to translate them again (because of the different placeholder system).

But for similar reason, we're keeping old strings on Transifex until we can move them to Fluent.

As a result, we need an additional branch on translation.git for base-browser translations that use Fluent, and an additional project to fetch them. This is intended to be a temporary thing, until we switch everything to Fluent.

@richard updating this will be an additional step for the relprep (sorry 😅 ).