Add experimental support for importing extension modules compiled for WASI

```
❯ ./run_wasi.sh -c "import importlib.machinery; print(importlib.machinery.EXTENSION_SUFFIXES)"
[]
```

See https://bytecodealliance.zulipchat.com/#narrow/stream/223391-wasm/topic/Python.20guest.20runtime.20and.20bindings/near/365948067 where this is being brought up.