Add non-component-coupled version of onMounted to @vue/reactivity and possibly other lifecycle APIs

[b-strauss]

Hi

@vue/reactivity is a great package to use as a foundation for a custom component based framework.
With the help of effectScope and onScopeDispose, one can build quite a robust system.

Unfortunately, if such a system tries to use existing reactivity libraries from the vue ecosystem, for instance VueUse, this will fail. VueUse is using lifecycle methods from vue that do not have a non-component-coupled version. Because of this, a system that is only built on @vue/reactivity, can not use it.

I propose to add these APIs to @vue/reactivity (like watch and onScopeDispose in the past) and make it more component system agnostic. When this would be done, other packages could follow and move more dependencies from vue to @vue/reactivity. This way, systems that build upon @vue/reactivity would benefit.

Possible APIs to consider:

• onBeforeMount
• onMounted
• onBeforeUnmount
• onUnmounted (solved with onScopeDispose)