feat: improve reactive checks

[pikax]

Instead of changing reactive object we just use a weak set to keep track on what objects are reactive

This allows to have arrays as reactive, bringing it closer to vue 3

Still need to:

• Extensive testing

Close #501, close #219

[antfu]

Should we hijack the array methods or just leave a caveat in the readme?

[pikax]

Is quite interesting idea, just thinking we might need to listen to other "events" to keep it consistent, for example, if you assign an array to an object, are we hijacking that array?

[antfu]

What do you mean "events"? If I recall correctly, Vue 2 hijacked the array's push/unshift, etc to achieve the reactivity updates.

[pikax]

when you add a new property to an object or when you change the property of the obj to an array

[antfu]

Oh, I see, I got what you mean. And yeah, if we handle the hijacking here and wrap the value with markReactive for newly added properties, they should be well covered I think.