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-parser/browser.js

@@ -0,0 +1,29 @@
+// Main entrypoint for ESM web browser environments. Avoids using Node.js
+// specific libraries, such as "path".
+//
+// TODO: figure out reasonable web equivalents for "resolve", "normalize", etc.
+import { camelCase, decamelize, looksLikeNumber } from './build/lib/string-utils.js'
+import { YargsParser } from './build/lib/yargs-parser.js'
+const parser = new YargsParser({
+  cwd: () => { return '' },
+  format: (str, arg) => { return str.replace('%s', arg) },
+  normalize: (str) => { return str },
+  resolve: (str) => { return str },
+  require: () => {
+    throw Error('loading config from files not currently supported in browser')
+  },
+  env: () => {}
+})
+
+const yargsParser = function Parser (args, opts) {
+  const result = parser.parse(args.slice(), opts)
+  return result.argv
+}
+yargsParser.detailed = function (args, opts) {
+  return parser.parse(args.slice(), opts)
+}
+yargsParser.camelCase = camelCase
+yargsParser.decamelize = decamelize
+yargsParser.looksLikeNumber = looksLikeNumber
+
+export default yargsParser