feat: underline markdown support (#2073)

* fix: no markdown support for underline #2072

client/libs/markdown-it-underline/index.js

@@ -0,0 +1,12 @@
+const renderEm = (tokens, idx, opts, env, slf) => {
+  const token = tokens[idx];
+  if (token.markup === '_') {
+    token.tag = 'u';
+  }
+  return slf.renderToken(tokens, idx, opts);
+}
+
+module.exports = (md) => {
+  md.renderer.rules.em_open = renderEm;
+  md.renderer.rules.em_close = renderEm;
+}