chore: Move to Typescript (#2783)

This PR moves the entire project to Typescript. Due to the ~1000 ignores this will lead to a messy codebase for a while, but the churn is worth it – all of those ignore comments are places that were never type-safe previously.

closes #1282

server/typings/fetch-with-proxy.d.ts

@@ -0,0 +1,3 @@
+declare module "fetch-with-proxy" {
+  export = fetch;
+}