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/utils/url.ts

@@ -0,0 +1,15 @@
+export function pathEscapeSegments(s) {
+  return s.split('/').map(encodeURIComponent).join('/');
+}
+
+function stripSlash(url) {
+  return url.endsWith('/') ? url.slice(0, -1) : url;
+}
+
+export function isUrl(url) {
+  try {
+    return stripSlash((new URL(url).href)).trim() === stripSlash(url).trim();
+  } catch {
+    return false;
+  }
+}