Add first version of targets command

[rylev]

Implements #115

This adds a wac targets subcommand for determining whether a given component targets a given world. See the README changes for how this command is supposed to be used.

The implementation is fairly naive, and I'm unsure if it's the best way to go about it. Because type checking relies on all types by populating in a Types collection, we load everything into Package which handles populating the Types collection. A Package requires a valid component, so we first convert the wit package into a component with wit-component. This does mean we have to dig into package to get the actual world id we care about though.

@peterhuene let me know if I'm missing something here, but I think this is probably the best way to do this for now.

In the future, we want to expose a programatic API so if we can figure out a way to populate a Types collection directly from a wit_parser::Resolve that might be the way to go.

[peterhuene]

Looks great, just some minor comments.

[fibonacci1729]

This looks great! Just need to resolve the one remaining comment about the positional arguments.