REFACTOR: Migrate markdown functionality in ES6

plugins/discourse-details/assets/javascripts/lib/discourse-markdown/details.js.es6

@@ -0,0 +1,25 @@
+import { registerOption } from 'pretty-text/pretty-text';
+
+function insertDetails(_, summary, details) {
+  return `<details><summary>${summary}</summary>${details}</details>`;
+}
+
+// replace all [details] BBCode with HTML 5.1 equivalent
+function replaceDetails(text) {
+  text = text || "";
+
+  while (text !== (text = text.replace(/\[details=([^\]]+)\]((?:(?!\[details=[^\]]+\]|\[\/details\])[\S\s])*)\[\/details\]/ig, insertDetails)));
+
+  // add new lines to make sure we *always* have a <p> element after </summary> and around </details>
+  // otherwise we can't hide the content since we can't target text nodes via CSS
+  return text.replace(/<\/summary>/ig, "</summary>\n\n").replace(/<\/details>/ig, "\n\n</details>\n\n");
+}
+
+registerOption((siteSettings, opts) => {
+  opts.features.details = true;
+});
+
+export function setup(helper) {
+  helper.whiteList('details.elided');
+  helper.addPreProcessor(text => replaceDetails(text));
+}