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/common.ts

@@ -0,0 +1,8 @@
+export function displayError(el, err) {
+  el.classList.remove('is-loading');
+  const errorNode = document.createElement('pre');
+  errorNode.setAttribute('class', 'ui message error markup-block-error');
+  errorNode.textContent = err.str || err.message || String(err);
+  el.before(errorNode);
+  el.setAttribute('data-render-done', 'true');
+}