feat(watch): support dot-delimited path in watch option

packages/runtime-core/__tests__/apiOptions.spec.ts

@@ -113,6 +113,7 @@ describe('api: options', () => {
     const spyB = jest.fn(returnThis)
     const spyC = jest.fn(returnThis)
     const spyD = jest.fn(returnThis)
+    const spyE = jest.fn(returnThis)
 
     let ctx: any
     const Comp = {
@@ -123,7 +124,10 @@ describe('api: options', () => {
           baz: {
             qux: 3
           },
-          qux: 4
+          qux: 4,
+          dot: {
+            path: 5
+          }
         }
       },
       watch: {
@@ -137,7 +141,8 @@ describe('api: options', () => {
         },
         qux: {
           handler: 'onQuxChange'
-        }
+        },
+        'dot.path': spyE
       },
       methods: {
         onFooChange: spyA,
@@ -175,6 +180,11 @@ describe('api: options', () => {
     await nextTick()
     expect(spyD).toHaveBeenCalledTimes(1)
     assertCall(spyD, 0, [5, 4])
+
+    ctx.dot.path++
+    await nextTick()
+    expect(spyE).toHaveBeenCalledTimes(1)
+    assertCall(spyE, 0, [6, 5])
   })
 
   test('watch array', async () => {