fix(reactivity): mutating a readonly ref nested in a reactive object should fail. (close #5042)

[LinusBorg]

When we have a readonly object nested in another reactive object, attempting to write to a property of that readonly object fails, as expected.

const state = reactive({
  subState: readonly(reactive({ msg: 'Hello' }))
})
state.subState.msg = 'Bye' // => fails as `subState` is readonly

But if we have a readonly ref nested in a reactive object, it successfully writes to it.

const state = reactive({
  msg: readonly(ref('Hello' ))
})
state.msg = 'Bye' // => succeeds even though `msg` is a readonly ref.

This PR fixes that edge case.

Open questions

• Is the behavior introduced by this PR the desired one?
• Is this a "risky fix" as it may break code that works in current user projects?

fix: #5042

[caozhong1996]

Such cases are extremely rare, you can expect no one will rely on it, so this change won't cause problems.
For me, I hope readonly is highest priority, because I don't want to memorize summary table.