fix(types): add a type-only differentiator to assist Mixin's type infer

[HcySunYang]

Fix: #3468

I noticed that when the mixins contain empty components:

defineComponent({
  mixins: [
    defineComponent({ methods: { foo() {} } }),
    // empty component
    defineComponent({})
  ]
})

Or they have the same type pattern:

defineComponent({
  mixins: [
    defineComponent({ props: [], methods: { foo() {} } }),
    // props type are same
    defineComponent({ props: [] })
  ]
})

Then the Mixin type is not inferred as the Unions type of the two components. So my idea is to add a type-only differentiator to help infer the type of Mixin.

[HcySunYang]

@pikax I updated the PR, can you give a new review