fix(watch): watch will trigger when added new keys using set

[pikax]

fix #467

[antfu]

If I have recalled right, Vue 3 doesn't offer the set api. As the reactivity systems underneath are different, I think the set api is useful and necessary in some situations with Vue 2. Should we add a note in the readme to mention this difference?

[pikax]

set is only to circumvent the limitation on adding a new key limitation on vue2. In vue3 there's no such limitation when using proxys.

We should add it to the readme.