fix(reactivity): avoid using WeakMap for IE compatibility

Using a WeakMap polyfill isn't ideal because the reason we tried to use
WeakMap was to work with non-extensible objects. However, WeakMap
polyfill for non-extensible objects are non-weak and could lead to
memory leaks.

The trade-off is that we remove support for `readonly()` on
non-extensible objects, which seems reasonable.

close #12837