Mark class prototype as read-only (#12115)

* initial code 2 fix the issue #2025

* Mark class prototype as read-only

* Update fixtures

* Fix failure

* Update Babel 8 fixtures

* Disable in loose mode

* Update fixtures

Co-authored-by: Nicolò Ribaudo <nicolo.ribaudo@gmail.com>

packages/babel-plugin-transform-jscript/test/fixtures/jscript/simple-class/output.js

@@ -1,4 +1,4 @@
-var Test = function () {
+var Test = /*#__PURE__*/babelHelpers.createClass(function () {
   function Test() {
     "use strict";
 
@@ -6,4 +6,4 @@ var Test = function () {
   }
 
   return Test;
-}();
+}());