Add a CLI subcommand for validating the target of a component

[peterhuene]

We should add a CLI subcommand for quickly validating the target of a component (any component, not just one from a composition).

Under the covers, it'll behave like the following (relatively silly) WAC document:

package composition:validation targets $WORLD;

export new $COMPONENT { ... }...;

Where $WORLD is the target world and $COMPONENT is the component being validated.

A suggested CLI UX:

wac validate --target <WORLD> <COMPONENT>

With example:

wac validate --target wasi:http/proxy my-proxy.wasm