Publish ESM (.mjs) entry point to npm

[motiz88]

While working on optimising the bundling of https://github.com/graphql/swapi-graphql — for reasons¹ — I discovered that it was importing things from graphql-relay-js, but that the latter, unlike graphql-js, only had a CommonJS entry point published to npm.

It might be desirable to add an ES module build target to this package, modeled on what graphql-js is already doing.

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¹ See https://twitter.com/motiz88/status/985110696668131328 - I'm investigating the claim that schema execution on the front-end is impractical due to graphql-js's footprint in terms of bundle size. swapi-graphql simply happens to be a good, accessible testbed for just that (thanks @IvanGoncharov for the tip! 🙏)

[motiz88]

I've pretty much completed this on my publish-mjs branch but hit webpack/webpack#7032 due to the use of Buffer in utils/base64.js. Looking for viable workarounds. UPDATE: I think I've found one, see #209.

[IvanGoncharov]

Sorry for the long delay.
We plan to merge this lib into graphql-js to provide the better maintenance and attention it deserves.
This should happen over the next few months and so closing this since graphql-js already provides mjs files.