build(package.json, rollup.config.js): fix esm build to meet Node.js …

…standard

To meet Node.js standard, 2 things should be fixed:

* ESM file should use `.mjs` extention
instead of `.esm.js`.
* `module` field in `package.json` is not standard way of Node.js. Use
`exports` field instead.

fix styled-components#649

package.json

@@ -27,8 +27,15 @@
     "colour"
   ],
   "main": "dist/polished.cjs.js",
-  "module": "dist/polished.esm.js",
+  "module": "dist/polished.mjs",
   "types": "lib/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./lib/index.d.ts",
+      "import": "./dist/polished.mjs",
+      "default": "./dist/polished.cjs.js"
+    }
+  },
   "sideEffects": false,
   "scripts": {
     "build": "yarn build:lib && yarn build:dist && yarn build:flow && yarn build:docs && yarn build:typescript",

rollup.config.js

@@ -44,7 +44,7 @@ const getBabelOptions = ({ useESModules }, targets) => ({
 export default [
   {
     input,
-    output: { file: `dist/${name}.esm.js`, format: 'esm' },
+    output: { file: `dist/${name}.mjs`, format: 'esm' },
     external,
     plugins: [sourceMaps(), resolve(), babel(getBabelOptions({ useESModules: true }))],
   },