Copy ESM modules into separate govuk-esm folder in package

Playing around with esm modules in govuk-frontend.

Based on original work by @domoscargin who did the initial investigation
into different approaches for this. This commit explores option 3 more.
#711 (comment)

package.json

@@ -3,6 +3,12 @@
   "description": "GOV.UK Frontend contains the code you need to start building a user interface for government platforms and services.",
   "version": "4.0.1",
   "main": "govuk/all.js",
+  "module": "govuk-esm/all.mjs",
+  "exports": {
+    "import": "./govuk-esm/all.mjs",
+    "require": "./govuk/all.js"
+  },
+  "sideEffects": false,
   "sass": "govuk/all.scss",
   "engines": {
     "node": ">= 4.2.0"