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.

types.d.ts

@@ -0,0 +1,4 @@
+declare module '*.svg' {
+  const value: string;
+  export default value;
+}