feat(reactivity): expose unref and shallowRef

test-dts/ref.test-d.ts

@@ -1,6 +1,5 @@
 import { expectType } from 'tsd'
-import { Ref, ref } from './index'
-import { isRef } from '@vue/reactivity'
+import { Ref, ref, isRef, unref } from './index'
 
 function foo(arg: number | Ref<number>) {
   // ref coercing
@@ -11,6 +10,9 @@ function foo(arg: number | Ref<number>) {
   if (isRef(arg)) {
     expectType<Ref<number>>(arg)
   }
+
+  // ref unwrapping
+  expectType<number>(unref(arg))
 }
 
 foo(1)