types(markRaw): keep the original type when unwrapping markRaw

[pikax]

related: #2981

When using markRaw on a reactive object the type will be unwrapped and it will lose it's type

describe('should support markRaw', () => {
  class Test<T> {
    item = {} as Ref<T>
  }
  const test = new Test<number>()
  const plain = {
    ref: ref(1)
  }

  const r = reactive({
    class: {
      raw: markRaw(test),
      reactive: test
    },
    plain: {
      raw: markRaw(plain),
      reactive: plain
    }
  })

  expectType<Test<number>>(r.class.raw)
  // @ts-expect-error it should unwrap
  expectType<Test<number>>(r.class.reactive)

  expectType<Ref<number>>(r.plain.raw.ref)
  // @ts-expect-error it should unwrap
  expectType<Ref<number>>(r.plain.reactive.ref)
})

[HcySunYang]

Has conflicts 😁

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[SinBirb]

Can this also be merged into @vue/composition-api, so we can use it with Vue 2.6 + Pinia?