Create a separate WASI container

[brettcannon]

It should make caching easier since the WASI toolchains will change more than the base dev container. It would also allow for https://github.com/python/cpython/tree/main/.devcontainer to have a separate WASI dev container which pre-builds everything.

[brettcannon]

If we don't use latest in the CPython repo then I think this idea actually isn't worth it as it will cause more work (at least for me) as I would then have to worry about pinning in this container and over in the CPython repo. I think I can still do a separate devcontainer in CPython by using a different command to run for the codespace pre-build.