Add a CLI subcommand for validating the target of a component

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:

```wac
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
```

With example:

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