fix(types): calling readonly() with ref() should return Readonly<Ref<T>> (#5212)

test-dts/ref.test-d.ts

@@ -10,7 +10,8 @@ import {
   toRef,
   toRefs,
   ToRefs,
-  shallowReactive
+  shallowReactive,
+  readonly
 } from './index'
 
 function plainType(arg: number | Ref<number>) {
@@ -236,6 +237,9 @@ expectType<Ref<string>>(p2.obj.k)
   expectType<Ref<number>>(x)
 }
 
+// readonly() + ref()
+expectType<Readonly<Ref<number>>>(readonly(ref(1)))
+
 // #2687
 interface AppData {
   state: 'state1' | 'state2' | 'state3'