N°5621 Move showdown 2.0.0-alpha1 to NPM

Current file header is : /*! showdown v 2.0.0-alpha1 - 25-09-2018 */

Official 2.0.0-alpha doesn't match neither on content nor date :(
Considering date we would be at 1.8.7, but it is also quite different (and has vulnerabilities)
In consequence switching to 2.0.0.

node_modules/yargs/browser.mjs

@@ -0,0 +1,7 @@
+// Bootstrap yargs for browser:
+import browserPlatformShim from './lib/platform-shims/browser.mjs';
+import {YargsFactory} from './build/lib/yargs-factory.js';
+
+const Yargs = YargsFactory(browserPlatformShim);
+
+export default Yargs;