Drop IE 11 support by default (#5090)

* Drop ie 11 support and move polyfills to a new package

* More useful directions for what entry point to use
https://github.com/facebook/create-react-app/pull/5090#discussion_r220313783

* Clear up what file this polyfill goes in
https://github.com/facebook/create-react-app/pull/5090#discussion_r220313980

* Polyfill `window`, not `global`

* Remove proxy polyfill file

packages/react-scripts/scripts/utils/createJestConfig.js

@@ -22,7 +22,7 @@ module.exports = (resolve, rootDir, isEjecting) => {
   // in Jest configs. We need help from somebody with Windows to determine this.
   const config = {
     collectCoverageFrom: ['src/**/*.{js,jsx}'],
-    setupFiles: [resolve('config/polyfills.js')],
+    setupFiles: ['react-app-polyfill/jsdom'],
     setupTestFrameworkScriptFile: setupTestsFile,
     testMatch: [
       '<rootDir>/src/**/__tests__/**/*.{js,jsx}',