Syntax for wasi-virt support

WAC should have syntax for creating [WASI-Virt](https://github.com/bytecodealliance/WASI-Virt) instances so that they can be composed with other instances.

Some hypothetical expression syntax:

```
virtualize {
fs: {
mounts: {
"/guest-path": "./host-path",
},
preopens: {
"/guest-path": "./host-path",
},
},
env: {
vars: {
"FOO": "BAR",
},
inherit: ["HOST-VAR"],
// or: inherit: *,
}
// other WASI-Virt configuration here
}
```

which results in an instance that exports the requisite WASI interfaces (instances).