fix(reactivity): ensure add/set on reactive collections return the proxy (fix: #2530)

[LinusBorg]

We currently return the raw target instead of the proxy on the following calls:

reactive(new Map()).set('a', 'a')
reactive(new WeakMap()).set({}, 'a')
reactive(new Map()).add('a')
reactive(new WeakMap()).add({})

This fix now returns the proxy

close: #2530

[github-actions]

Size report

Path	Size
vue.global.prod.js	40.15 KB (+0.02% 🔺)
runtime-dom.global.prod.js	26.43 KB (+0.02% 🔺)
size-check.global.prod.js	15.95 KB (+0.03% 🔺)