Add Linux support

[Flerex]

URLRequest object and other networking components have been separated into a different module (i.e. FoundationNetworking) in Linux-based systems and are not available anymore on Foundation.

This PR includes this new module, checking first whether it's available so that it still works in the Darwin kernel.

See: https://forums.swift.org/t/pitch-move-urlsession-to-new-foundationnetworking-module/14002?page=2

[Flerex]

BTW: You had it right before, it's "a UTF-8", not "an UTF-8" because the letter U has a consonant sound there (it sounds more like a "y").

[mw99]

Oh that change just happened? Kinda a big thing to separate that out of Foundation only on linux. Doesn't that break tons of legacy code? I will see how this develops, looks kinda strange to me atm. Thanks for the typo though.