Fixes #1049.

An example to reproduce the problem:

<script setup>
import { h } from 'vue'

const Mixin = {
  computed: {
    a() {
      return 1
    },
  },
}

const Comp1 = {
  mixins: [Mixin],
  computed: {
    b() {
      return 2
    },
  },
  render() {
    return h('div', 'comp 1')
  },
}

const Comp2 = {
  mixins: [Mixin],
  render() {
    return h('div', 'comp 2')
  },
}
</script>

<template>
  <div>
    <Comp1 />
    <hr>
    <Comp2 />
  </div>
</template>

In this example, inspecting Comp2 shows a computed property called b, even though Comp2 doesn't have a property called b. That property has leaked from Comp1, via the mixin.

In mergeOptions, we currently have this line:

to[key] = from[key]

That's using the original object from the mixin, not a copy. This is the underlying problem. Later we hit this line:

Object.assign(to[key], from[key])

That modifies to[key], adding properties from from[key]. In my earlier example, to[key] is the computed object from the mixin, and this copies in the property b from Comp2.

The same problem applies to inject.