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/markup/math.ts

@@ -0,0 +1,47 @@
+import {displayError} from './common.ts';
+
+function targetElement(el) {
+  // The target element is either the current element if it has the
+  // `is-loading` class or the pre that contains it
+  return el.classList.contains('is-loading') ? el : el.closest('pre');
+}
+
+export async function renderMath() {
+  const els = document.querySelectorAll('.markup code.language-math');
+  if (!els.length) return;
+
+  const [{default: katex}] = await Promise.all([
+    import(/* webpackChunkName: "katex" */'katex'),
+    import(/* webpackChunkName: "katex" */'katex/dist/katex.css'),
+  ]);
+
+  const MAX_CHARS = 1000;
+  const MAX_SIZE = 25;
+  const MAX_EXPAND = 1000;
+
+  for (const el of els) {
+    const target = targetElement(el);
+    if (target.hasAttribute('data-render-done')) continue;
+    const source = el.textContent;
+
+    if (source.length > MAX_CHARS) {
+      displayError(target, new Error(`Math source of ${source.length} characters exceeds the maximum allowed length of ${MAX_CHARS}.`));
+      continue;
+    }
+
+    const displayMode = el.classList.contains('display');
+    const nodeName = displayMode ? 'p' : 'span';
+
+    try {
+      const tempEl = document.createElement(nodeName);
+      katex.render(source, tempEl, {
+        maxSize: MAX_SIZE,
+        maxExpand: MAX_EXPAND,
+        displayMode,
+      });
+      target.replaceWith(tempEl);
+    } catch (error) {
+      displayError(target, error);
+    }
+  }
+}