feat(runtime-core): export currentInstance methods

[sibbng]

Close #4611

Hi, I'm maintainer of reactivue. I rewrote it from scratch and managed to shrink it down to ~150 lines by using EffectScope API. But to support Vue plugins like pinia, @vueuse/head I needed to access setCurrentInstance method.

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[LinusBorg]

I'm not sure we feel comfortable exposing hooks into these internals.

Can you share a bit more about the use case and what problem this solves?

[crutchcorn]

@LinusBorg I actually wrote a lot about that here

#4611

That said, how about we export this under a namespace similar to how we export SSR utils?