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/index.ts

@@ -0,0 +1,5 @@
+import './polyfills.ts';
+import '@github/relative-time-element';
+import './origin-url.ts';
+import './overflow-menu.ts';
+import './absolute-date.ts';