fix(reactivity): scheduled effect should not execute if stopped

fix #910

packages/reactivity/__tests__/effect.spec.ts

@@ -678,6 +678,28 @@ describe('reactivity/effect', () => {
     expect(dummy).toBe(3)
   })
 
+  it('stop with scheduler', () => {
+    let dummy
+    const obj = reactive({ prop: 1 })
+    const queue: (() => void)[] = []
+    const runner = effect(
+      () => {
+        dummy = obj.prop
+      },
+      {
+        scheduler: e => queue.push(e)
+      }
+    )
+    obj.prop = 2
+    expect(dummy).toBe(1)
+    expect(queue.length).toBe(1)
+    stop(runner)
+
+    // a scheduled effect should not execute anymore after stopped
+    queue.forEach(e => e())
+    expect(dummy).toBe(1)
+  })
+
   it('events: onStop', () => {
     const onStop = jest.fn()
     const runner = effect(() => {}, {