Support native ESM via .mjs.

Relates to graphql#425.

- Swapped deprecated babel-preset-es2015 for babel-preset-env.
- Added package.json engines field.
- Configured babel-preset-env to target the Node.js version defined in package.json engines field.
- Kept Babel at v6, but moved config into a v7 ready .babelrc.js file for dynamic config via env.
- Tidied the package.json scripts, added an ESM build step that generates .mjs dist files. Follow the lead of graphql/graphql-js#1244, although I do things quite different for my own projects with Babel v7.
- Updated package.json main field for ESM/CJS cross compatibility.
- Added a package.json module field for tree-shaking bundlers.

.babelrc.js

@@ -0,0 +1,15 @@
+module.exports = {
+  presets: [
+    [
+      'env',
+      {
+        modules: process.env.ESM ? false : 'commonjs',
+        targets: {
+          node: require('./package.json').engines.node.substring(2), // Strip `>=`
+        },
+      },
+    ],
+  ],
+  plugins: ['transform-class-properties', 'transform-flow-strip-types'],
+  ignore: ['__tests__'],
+};

package.json

@@ -19,7 +19,11 @@
     "middleware",
     "api"
   ],
-  "main": "dist/index.js",
+  "engines": {
+    "node": ">=4"
+  },
+  "main": "dist/index",
+  "module": "dist/index.mjs",
   "directories": {
     "lib": "./dist"
   },
@@ -34,26 +38,25 @@
   },
   "babel": {
     "presets": [
-      "es2015"
-    ],
-    "plugins": [
-      "transform-class-properties",
-      "transform-flow-strip-types"
+      "./.babelrc.js"
     ]
   },
   "scripts": {
-    "prepublish": ". ./resources/prepublish.sh",
     "test": "npm run lint && npm run check && npm run testonly",
     "testonly": "mocha $npm_package_options_mocha",
     "lint": "eslint src",
     "prettier": "prettier --write 'src/**/*.js'",
     "check": "flow check",
-    "build": "rm -rf dist/* && babel src --ignore __tests__ --out-dir dist && npm run build:flow",
+    "build": "npm run build:clean && npm run build:esm && npm run build:cjs && npm run build:flow",
+    "build:clean": "rm -rf dist/*",
+    "build:esm": "ESM=1 babel src -d dist/esm && for file in $(find dist/esm -name '*.js'); do mv \"$file\" `echo \"$file\" | sed 's/dist\\/esm/dist/g; s/.js$/.mjs/g'`; done && rm -rf dist/esm",
+    "build:cjs": "babel src -d dist",
     "build:flow": "find ./src -name '*.js' -not -path '*/__tests__*' | while read filepath; do cp $filepath `echo $filepath | sed 's/\\/src\\//\\/dist\\//g'`.flow; done",
     "watch": "node resources/watch.js",
     "cover": "babel-node node_modules/.bin/isparta cover --root src --report html node_modules/.bin/_mocha -- $npm_package_options_mocha",
     "cover:lcov": "babel-node node_modules/.bin/isparta cover --root src --report lcovonly node_modules/.bin/_mocha -- $npm_package_options_mocha",
     "preversion": "npm test",
+    "prepublish": ". ./resources/prepublish.sh",
     "start": "babel-node examples/index.js"
   },
   "dependencies": {
@@ -69,7 +72,7 @@
     "babel-plugin-transform-class-properties": "6.24.1",
     "babel-plugin-transform-flow-strip-types": "6.22.0",
     "babel-plugin-transform-runtime": "^6.23.0",
-    "babel-preset-es2015": "6.24.1",
+    "babel-preset-env": "^1.6.1",
     "babel-register": "^6.26.0",
     "babel-runtime": "^6.26.0",
     "body-parser": "1.18.2",