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.

[brettcannon]

This might make builds somewhat faster by layering a base container and then WASI on top of it (see #8).

[brettcannon]

I don't think this is worth the overhead right now as changing the base devcontainer would then require updating the WASI container unless we start using labels more (e.g. labeling a container image as meant for Python 3.15 or something).

[brettcannon]

I asked for feedback on what people wanted for WASI and this didn't come up, so I'm not going to worry about it.