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/polyfills.ts

@@ -0,0 +1,17 @@
+try {
+  // some browsers like PaleMoon don't have full support for Intl.NumberFormat, so do the minimum polyfill to support "relative-time-element"
+  // https://repo.palemoon.org/MoonchildProductions/UXP/issues/2289
+  new Intl.NumberFormat('en', {style: 'unit', unit: 'minute'}).format(1);
+} catch {
+  const intlNumberFormat = Intl.NumberFormat;
+  Intl.NumberFormat = function(locales, options) {
+    if (options.style === 'unit') {
+      return {
+        format(value) {
+          return ` ${value} ${options.unit}`;
+        },
+      };
+    }
+    return intlNumberFormat(locales, options);
+  };
+}