Support for targeting multiple worlds

It would be great to support the ability to create a component that can target the union of multiple worlds in the cases where the user cannot edit the wit file to create a third world that includes the target worlds. An example of this would be an SDK built on top of a wit that bundles the wit in an npm package and the user application wanting to target their world where there would be 2 distinct `wit` folders.

This request is motivated by the ability to do that in `componentize-py` (the [commit](https://github.com/bytecodealliance/componentize-py/commit/20787522a8822ae0ae7d9cdb18c6163cde9e7aac) that added the support).