Support .mjs as file extension for es modules

[mweststrate]

Summary

Support .mjs as file extension for es modules assuming they have been transpiled properly that is, like webpack does.

Example setup: https://github.com/mobxjs/mobx/pull/1131/files

Should solve #59

Beyond that I

Test plan

I have no clue what I am doing, nor do I have any experience with RN so this is just a proposal 😁

Testing can be done by dropping the following to files in <modules folder>/mjstest

package.json

{
   "version": "1.0.0",
   "name": "mjstest"
   "module": "index.mjs"
}

index.mjs

function log() {
  console.log("hello from .mjs")
}

export { log }

and than in an app use it:

import { log } from "mjstest"

log() // should log something

I realize this is the most pathetic attempt of a PR ever. So feel free to close it, but I hope it might have some use as a starting point for support the .mjs extension :)

[mweststrate]

Closing for now, until .mjs has become a bit more mainstream. For further discussion see #59

[bkaney]

Looking that this 2 years later, perhaps it makes sense to support .mjs now?