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.

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 =