Add ESM build for tree-shaking

[sheepsteak]

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

Prior discussion of this in #343 and #344.

• Produce build with ESM modules in it into es folder
• Fix require in tests by using esModuleInterop into tsconfig.json
• Add module field so that tooling can find the ESM build

Once merged the ESM build will just get done alongside the ES5 build. I did npm pack to make sure the es folder will get included.

I linked this version into an AWS Lambda function where I'm only importing makeExecutableSchema and nothing else from graphql-tools. I saw a 100KiB reduction in a production build size.

[apollo-cla]

@sheepsteak: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[yaacovCR]

Closed in favor of #1320