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/webcomponents/origin-url.ts

@@ -0,0 +1,22 @@
+// Convert an absolute or relative URL to an absolute URL with the current origin. It only
+// processes absolute HTTP/HTTPS URLs or relative URLs like '/xxx' or '//host/xxx'.
+// NOTE: Keep this function in sync with clone_script.tmpl
+export function toOriginUrl(urlStr) {
+  try {
+    if (urlStr.startsWith('http://') || urlStr.startsWith('https://') || urlStr.startsWith('/')) {
+      const {origin, protocol, hostname, port} = window.location;
+      const url = new URL(urlStr, origin);
+      url.protocol = protocol;
+      url.hostname = hostname;
+      url.port = port || (protocol === 'https:' ? '443' : '80');
+      return url.toString();
+    }
+  } catch {}
+  return urlStr;
+}
+
+window.customElements.define('origin-url', class extends HTMLElement {
+  connectedCallback() {
+    this.textContent = toOriginUrl(this.getAttribute('data-url'));
+  }
+});