Add ?uselang= support to toolforge_i18n

I initially intended to keep this limited to lexeme-forms, but as soon
as I tried to use this library in Wikidata Image Positions, I realized
how much more annoying it is to test the translations without this
feature. And really, I think it makes sense in general – it’s just
mirroring MediaWiki, after all. (Note that the argument is also a
MediaWiki language code, not a BCP-47 language code.)

The precedence in the tool’s custom implementation – ?uselang above
session above Accept-Language header – is a bit awkward, but I don’t
think that can be helped unless I want to add some sort of language
preference support to the default implementation, and I’m not sure I
want to do that yet.