EcmaScript Modules can (actually must, but easy to wrap the sync stuff in a promise) resolve asynchronously. While it's in no way a blocker for ESM support in Jest, it would be nice to add support for async transforms to be used.

This logic should for now not impact anything in Jest beyond @jest/transform as Jest wouldn't use it until ESM support is in place.

I'm thinking we can add a .transformAsync and .transformSourceAsync to the ScriptTransformer class.

There are some checks and verifications that process is a function - we need to make sure there is one clear code path for sync and one for async transformation that verify the provided reporter implements the correct interface.

Sort of reopening #5556 and all its linked issues.