Support superset libraries (upstream using Mercury)

[MulverineX]

Related RFC: openwebf/rfc#7

• API for adding EventTarget/Global context modules (Add an global API for the EventTarget registration #9)
• API for adding custom FFI bindings
• API for using custom binding binaries
• Release JS codegen & bindings as an externalized library
• JS codegen API & C++ interface(s) for extending the bindings before compilation
• Refactor WebF to use mercury