[PM-20247] Initialize user-core library (#15029)

* Initialize user-core library

* Run `npm install`

* Fix patched generator bug

libs/user-core/src/index.ts

@@ -0,0 +1,9 @@
+import { Opaque } from "type-fest";
+
+/**
+ * The main identifier for a user. It is a string that should be in valid guid format.
+ *
+ * You should avoid `as UserId`-ing strings as much as possible and instead retrieve the {@see UserId} from
+ * a valid source instead.
+ */
+export type UserId = Opaque<string, "UserId">;