Allow unversioned project to select a specific version of a versioned library

[timotheeguerin]

fix #323

in library:

@versioned("1" | "2")
namespace VersionedLib {
  model Foo {
    name: string;
    @added("2") age: int32;
  }
}

in project use version 1 of library

@versionedDependency(VersionedLib, "1")
namespace MyService;

model Test extends VersionedLib.Foo {}

[markcowl]

I think it would be best to get Brian's feedback about the toTypescript projection before merging, but LGTM.

[bterlson]

Excellent!