Upgrade to ECMAScript 2020

This is needed in order to use `globalThis`, see #1894.
It also made it possible to remove some old polyfills and fallbacks.

.eslintrc.js

@@ -5,7 +5,10 @@ module.exports = {
   },
   env: {
     node: true,
-    es6: true
+    es2020: true
+  },
+  parserOptions: {
+    sourceType: 'module'
   },
   rules: {
     'no-console': 'warn',
@@ -57,10 +60,5 @@ module.exports = {
     // ECMAScript 6 //
     //--------------//
     'no-var': 'error'
-  },
-  parserOptions: {
-    sourceType: 'module',
-    ecmaVersion: 6,
-    ecmaFeatures: {}
   }
 };

README.markdown

@@ -79,7 +79,7 @@ There are a few Mustache behaviors that Handlebars does not implement.
 Supported Environments
 ----------------------
 
-Handlebars has been designed to work in any ECMAScript 7 (2016) environment. This includes
+Handlebars has been designed to work in any ECMAScript 2020 environment. This includes
 
 - Node.js
 - Chrome

lib/.eslintrc.js

@@ -1,3 +1,4 @@
+/* eslint-env node */
 module.exports = {
   env: {
     // Handlebars should run natively in the browser

lib/handlebars/compiler/javascript-compiler.js

@@ -128,6 +128,7 @@ JavaScriptCompiler.prototype = {
       this.decorators.push('return fn;');
 
       if (asObject) {
+        // eslint-disable-next-line no-new-func
         this.decorators = Function.apply(this, [
           'fn',
           'props',
@@ -260,7 +261,7 @@ JavaScriptCompiler.prototype = {
     if (asObject) {
       params.push(source);
 
-      return Function.apply(this, params);
+      return Function.apply(this, params); // eslint-disable-line no-new-func
     } else {
       return this.source.wrap([
         'function(',

lib/handlebars/utils.js

@@ -30,23 +30,10 @@ export function extend(obj /* , ...source */) {
 export let toString = Object.prototype.toString;
 
 // Sourced from lodash
-// https://github.com/bestiejs/lodash/blob/master/LICENSE.txt
-/* eslint-disable func-style */
-let isFunction = function(value) {
+// https://github.com/lodash/lodash/blob/4.17.21/LICENSE
+export function isFunction(value) {
   return typeof value === 'function';
-};
-// fallback for older versions of Chrome and Safari
-/* istanbul ignore next */
-if (isFunction(/x/)) {
-  isFunction = function(value) {
-    return (
-      typeof value === 'function' &&
-      toString.call(value) === '[object Function]'
-    );
-  };
 }
-export { isFunction };
-/* eslint-enable func-style */
 
 /* istanbul ignore next */
 export const isArray =