Add service response types for standardizing returns.

Typescript is currently having difficulty parsing Service class return
types. Providing more explicit return types should improve the
situation.

src/lib/server/services/service.types.ts

@@ -0,0 +1,12 @@
+type ServiceResponseSuccess<T, D> = {
+  status: T;
+  data?: D;
+};
+type ServiceResponseFailure<T, E> = {
+  status: T;
+  code?: E;
+  error?: Error | string;
+};
+export type ServiceResponse<D, E, S = "ok", F = "failure"> =
+  | ServiceResponseSuccess<S, D>
+  | ServiceResponseFailure<F, E>;