REFACTOR: Move the discourse-markdown engine out of the addon repo

The reasoning here is that we want to make a unique bundle for this
folder and the default approach is to include everything in `addon`.

app/assets/javascripts/pretty-text/engines/discourse-markdown/code.js

@@ -0,0 +1,58 @@
+// we need a custom renderer for code blocks cause we have a slightly non compliant
+// format with special handling for text and so on
+
+const TEXT_CODE_CLASSES = ["text", "pre", "plain"];
+
+function render(tokens, idx, options, env, slf, md) {
+  let token = tokens[idx],
+    info = token.info ? md.utils.unescapeAll(token.info) : "",
+    langName = md.options.discourse.defaultCodeLang,
+    className,
+    escapedContent = md.utils.escapeHtml(token.content);
+
+  if (info) {
+    // strip off any additional languages
+    info = info.trim().split(/\s+/g)[0];
+  }
+
+  const acceptableCodeClasses = md.options.discourse.acceptableCodeClasses;
+  if (
+    acceptableCodeClasses &&
+    info &&
+    acceptableCodeClasses.indexOf(info) !== -1
+  ) {
+    langName = info;
+  }
+
+  className =
+    TEXT_CODE_CLASSES.indexOf(info) !== -1
+      ? "lang-nohighlight"
+      : "lang-" + langName;
+
+  return `<pre><code class="${className}">${escapedContent}</code></pre>\n`;
+}
+
+export function setup(helper) {
+  helper.registerOptions((opts, siteSettings) => {
+    opts.defaultCodeLang = siteSettings.default_code_lang;
+    opts.acceptableCodeClasses = (siteSettings.highlighted_languages || "")
+      .split("|")
+      .concat(["auto", "nohighlight"]);
+  });
+
+  helper.whiteList({
+    custom(tag, name, value) {
+      if (tag === "code" && name === "class") {
+        const m = /^lang\-(.+)$/.exec(value);
+        if (m) {
+          return helper.getOptions().acceptableCodeClasses.indexOf(m[1]) !== -1;
+        }
+      }
+    },
+  });
+
+  helper.registerPlugin((md) => {
+    md.renderer.rules.fence = (tokens, idx, options, env, slf) =>
+      render(tokens, idx, options, env, slf, md);
+  });
+}