New `foreign` syntax

[ospencer]

This work likely won't be done until our module system rewrite. This is what declaring foreign values would look like:

@externalName("wasi_unstable")
foreign module WasiUnstable {
  @externalName("fd_write")
  let fdWrite: annotation
  ...
}

In the future when toplevel wasm imports exist, they would use the toplevel foreign keyword, similar to how it is done now.