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/y18n/build/lib/platform-shims/node.js

@@ -0,0 +1,19 @@
+import { readFileSync, statSync, writeFile } from 'fs';
+import { format } from 'util';
+import { resolve } from 'path';
+export default {
+    fs: {
+        readFileSync,
+        writeFile
+    },
+    format,
+    resolve,
+    exists: (file) => {
+        try {
+            return statSync(file).isFile();
+        }
+        catch (err) {
+            return false;
+        }
+    }
+};