propertyDefaults for observable class fields

Adds propertyDefaults support to observable class fields

class MyArray extends ObservableArray {

  foo = 4;

  static get propertyDefaults() {
    return type.maybeConvert(String);
  }
}

const anArray = new MyArray();

console.log(anArray.foo); // '4'

anArray.on('foo', (ev, newVal, oldVal) => {
  console.log(newVal); // -> '10'
});

anArray.set(foo, 10);

#85