feat(runtime-core): api to detach watchers and computed values from component lifecycle

[unbyte]

related: #1532

watch and wathEffect

option lifecycle: 'static' | 'instance', default to instance

with the static lifecycle, effects created inside a setup will still live after the component instance unmounted.

const stop = watchEffect(()=>{
        // ...
      }, { lifecycle: 'static' })

// in dev mode, a warn will be output to remind users to stop effects manually

staticComputed

in order to not break current behavior, provide a new api staticComputed which can create computed values of static lifecycle.

users can stop effects manually by call stop method on computed value

const someValue = ref(0)
const val = staticComputed(() => someValue.value + 1)
val.value // 1
val.stop() // stop the effect inside computed value

close #1532

[ZhangJian-3ti]

vuejs/rfcs#212
According to this RFC, I think it's a better way to resolve such issues.

[unbyte]

@ZhangJian-3ti you are right 😁

[unbyte]

wait, this rfc doesn't point out the problem that this PR wants to solve. I will keep it open until the discussion is clear.