Convert frontend code to typescript (#31559)

None of the frontend js/ts files was touched besides these two commands
(edit: no longer true, I touched one file in
61105d0618
because of a deprecation that was not showing before the rename).

`tsc` currently reports 778 errors, so I have disabled it in CI as
planned.

Everything appears to work fine.

web_src/js/modules/dirauto.ts

@@ -0,0 +1,40 @@
+import {isDocumentFragmentOrElementNode} from '../utils/dom.ts';
+
+// for performance considerations, it only uses performant syntax
+function attachDirAuto(el) {
+  if (el.type !== 'hidden' &&
+      el.type !== 'checkbox' &&
+      el.type !== 'radio' &&
+      el.type !== 'range' &&
+      el.type !== 'color') {
+    el.dir = 'auto';
+  }
+}
+
+export function initDirAuto() {
+  const observer = new MutationObserver((mutationList) => {
+    const len = mutationList.length;
+    for (let i = 0; i < len; i++) {
+      const mutation = mutationList[i];
+      const len = mutation.addedNodes.length;
+      for (let i = 0; i < len; i++) {
+        const addedNode = mutation.addedNodes[i];
+        if (!isDocumentFragmentOrElementNode(addedNode)) continue;
+        if (addedNode.nodeName === 'INPUT' || addedNode.nodeName === 'TEXTAREA') attachDirAuto(addedNode);
+        const children = addedNode.querySelectorAll('input, textarea');
+        const len = children.length;
+        for (let childIdx = 0; childIdx < len; childIdx++) {
+          attachDirAuto(children[childIdx]);
+        }
+      }
+    }
+  });
+
+  const docNodes = document.querySelectorAll('input, textarea');
+  const len = docNodes.length;
+  for (let i = 0; i < len; i++) {
+    attachDirAuto(docNodes[i]);
+  }
+
+  observer.observe(document, {subtree: true, childList: true});
+}