feat(runtime-core): hydration internal flag

[CyberAP]

Adds an internal hydration flag for created, beforeMount and mounted lifecycles, removes after the mounted. Closes#1723

[yyx990803]

Wondering if this is still relevant as there seems to be no one else raising similar needs. I'm removing this from 3.2 for now since I think we may have better consistency exposing isSSR and isHydrating via global callable methods, as part of 3.3 (which will focus on SSR).

[CyberAP]

I am strongly relying on hydration detection and forced rehydration mechanism up to this day in Vue 2. The implementation doesn't really matter that much to me, its more important that I can have a reliable way to detect a hydration in Vue 3. Looking forward for Vue 3.3!

[KaelWD]

We use this in vuetify as well.