esmock: Node 16.12+ breaks esmock

I can confirm that the issue has been resolved 👍

Two changes fix this package for the latest versions of node,

1. The module loader “getSource” definition must be exported as “load”. node shows this in the test process “(node:14251) DeprecationWarning: Obsolete loader hook(s) supplied and will be ignored: getSource”. I searched for ways to conditionally export “getSource” or “load” depending upon the node version but didn’t find a way to it.
2. The value returned by “load” needs to include a property format: 'module' or an error is thrown.

---

Here’s what I propose,

• release a minor version, exporting both ‘load’ and ‘getSource’ hooks. The deprecation warning will show when this package gets used,
• release a new major version, removing ‘getSource’ and updating the README with some information about that

Maybe “getSource” should be kept rather than removed, I’m not sure.

@mroderick do you have any input?

cc @Swivelgames feel free to give your opinion or ignore as well.