Provide .mjs entry for native ESM

[jaydenseric]

This will make it possible for packages dependant on graphql to in turn do the same for their consumers, without encountering babel/babel#7294.

Node.js with --experimental-modules can run ESM in .mjs files.

Essentially, package.json needs to go from this:

{
  "main": "index.js",
  "module": "module/index.js"
}

To this:

{
  "main": "index",
  "module": "index.mjs"
}

Sibling index.js (CJS) and index.mjs (ESM) files need to be published. Node.js will load the relevant file, looking up .mjs before .js for main if --experimental-modules is enabled.

[leebyron]

I'm happy to entertain PRs that add this functionality to the build process. We already produce module files, which are used by rollup and other ES6 aware bundlers. Is all that necessary is changing file extension?

[leebyron]

@jaydenseric I just put up a PR which seeks to support this case - would you might helping out with a test case? Even if you want to just pull down the PR and run the build to see if the resulting output solves things for you