This commit adds the js/wasm architecture to the os package.

Access to the actual file system is supported through Node.js.

Updates #18892

Change-Id: I6fa642fb294ca020b2c545649d4324d981aa0408
Reviewed-on: https://go-review.googlesource.com/109977Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>