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(',