feat: add file routing option

examples/custom/server/trpc/routers/index.ts

@@ -0,0 +1,10 @@
+import { router } from '../trpc'
+import { todoRouter } from './todo'
+import { userRouter } from './user'
+
+export const appRouter = router({
+  todo: todoRouter,
+  user: userRouter
+})
+
+export type AppRouter = typeof appRouter