esm: loader chaining

This is stage 1 of what I'm calling "Project: Make Loaders Usable". In future stages I will be working on such fun things as:

• Figuring out the context parameter of loader hooks
• Auditing how hooks behave as part of chaining (for example, transformSource doesn't need to short circuit)
• Exploring APIs to allow non-chaining composition of loaders
• Loader threading

cc @nodejs/modules-active-members

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines