buffer: graduate File from experimental and expose as global

Undici has been using the native File class by default for over 3 months now, and we haven't had a single issue reported about it or using it with fetch/FormData. It's a very small wrapper around Blob too, I don't personally see much reason in keeping a class with 2 additional properties experimental.

I based this commit heavily on jasnell's which made Blob global, lmk if I'm missing something