fix(runtime-core): watching multiple sources: computed

[Kingbultsea]

fix: #3068

[Kingbultsea]

fix: #3068

fix: #3059

[HcySunYang]

This PR has a flaw, consider the following:

const foo = ref([1])

watch(foo, () => {
  console.log('should trigger')
})

foo.value = foo.value.slice()

with your PR, resulting in not triggering the cb correctly. Here we should check whether the source of observation is really an array.

[LinusBorg]

@HcySunYang Can you have another look here? seems your requested changes were adressed`

[HcySunYang]

@LinusBorg I'm not sure if this is really reasonable, even if it works correctly.

[HcySunYang]

@Kingbultsea Could you please resolve the conflict?