Enable std::filesystem on libc++ on Windows

tor-browser!829 (closed) could use std::filesystem to avoid us using Win32 APIs with all their shenanigans.

Currently, we don't have it on Windows because Mozilla used to disable it.

In 116, they had to enable it because of LLVM updates (fstream now requires std::filesystem, in LLVM's implementation).

Tom said we can take that change already.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information