feat: Add stdout/stderr Fd class

[kateinoigakukun]

stdout/stderr should provide proper rights and filetype because wasi-libc still respects them in fcntl.

[bjorn3]

I intentionally didn't add stdin and stdout types because there is no stdin/stdout in the browser, so one way or another the user of browser_wasi_shim will have to make their own class to read/write from their own location. For example the rustc.html example uses a class that interacts with a terminal emulated by xterm.js.

[kateinoigakukun]

Okay, sounds reasonable to me. Then does it make sense to export wasi_defs.js from this package to allow users to implement Fd interfaces using some classes in the module (e.g. Filestat, Fdstat)?

[bjorn3]

Yeah, exposing those types should be fine.

[kateinoigakukun]

Closing in favor of exposing types in another PR