fix(runtime-core): return the exposeProxy from mount

[skirtles-code]

When using expose or defineExpose, the 'public' instance (exposeProxy) is used for template refs, $parent and $root. The instance returned by mount is currently still the internal instance.

const vm = createApp({
  setup(_, { expose }) {
    expose({ a: 1 })
    return () => h('div')
  }
}).mount('#app')

console.log(vm.a) // undefined as vm is the internal instance

This PR changes mount to return the exposeProxy.

I couldn't find any mention of mount in either the RFC or the associated discussion thread:

• RFC: https://github.com/vuejs/rfcs/blob/master/active-rfcs/0042-expose-api.md
• Discussion: Expose API discussion (#343) rfcs#345

I'm assuming it was simply overlooked, but if there is a good reason why mount should return the internal instance then it should probably be added to that thread.

I'm not sure whether this might impact tooling or testing, which might be relying on using mount to get to the internals of a <script setup> component. While I wouldn't necessarily call this a 'breaking change', I think it might be considered breaking enough to need to wait for 3.3.