Add Babel polyfill provider ES shims

Rollup will log a warning for code requiring polyfills:

> (!) Unresolved dependencies
> https://rollupjs.org/troubleshooting/#warning-treating-module-as-external-dependency
> error-cause/Error (imported by "src/govuk/errors/index.mjs")

Babel will also log a warning:

> Some polyfills have been added but are not present in your dependencies.
>
> Please run one of the following commands:
> 	npm install --save error-cause@^1.0.1
> 	yarn add error-cause@^1.0.1

See ES shims on GitHub: https://github.com/es-shims

packages/govuk-frontend/babel.config.js

@@ -5,6 +5,7 @@
  */
 module.exports = function (api) {
  const isBrowser = !api.env('test')
+ const isProduction = !api.env('development')
 
  // Apply Browserslist environment for supported targets
  // https://github.com/browserslist/browserslist#configuring-for-different-environments
@@ -33,6 +34,19 @@ module.exports = function (api) {
  return !isPrivate && isDocumentation
  }
  },
+ plugins: [
+ [
+ 'polyfill-es-shims',
+ {
+ // Add logging for required polyfills
+ debug: isProduction,
+
+ // Replace unsupported code with polyfills
+ // without modifying window globals
+ method: 'usage-pure'
+ }
+ ]
+ ],
  presets: [
  [
  '@babel/preset-env',

packages/govuk-frontend/package.json

@@ -73,6 +73,7 @@
  "@rollup/plugin-replace": "^5.0.5",
  "@rollup/plugin-terser": "^0.4.4",
  "autoprefixer": "^10.4.16",
+ "babel-plugin-polyfill-es-shims": "^0.10.0",
  "cssnano": "^6.0.1",
  "cssnano-preset-default": "^6.0.1",
  "govuk-prototype-kit": "^13.13.6",