Add mergeProps

[skirtles-code]

I don't think mergeProps is documented anywhere.

I'm pretty sure it's intended to be a public part of the API. It's certainly discussed that way in this RFC:

https://github.com/vuejs/rfcs/blob/master/active-rfcs/0031-attr-fallthrough.md

createVNode, cloneVNode and isVNode might also need documenting, though I'm less clear whether they're intended for public consumption.

https://github.com/vuejs/vue-next/blob/aa06b1034d8268fa15cb6b4b6916440701238b2d/packages/runtime-core/src/index.ts#L56