Add a value property getter & setter to the key module/observable

[chasenlehara]

This adds an API for getting & setting the value of the observation returned by the key module:

import keyObservable from "can-simple-observable/key/key";

const outer = {inner: {key: "hello"}};
const observable = keyObservable(outer, "inner.key");

observable.value; // is "hello"

observable.value = "goodbye";
observable.value; // is now "goodbye"

This PR doesn’t include docs because there are no docs for the key module (it’s used by can-value and will be mentioned there).

Related to canjs/can-observation#130

[chasenlehara]

The tests are failing right now because can-observation uses .value internally, which causes infinite recursion, but that should be fixed after canjs/can-observation#135 is merged (and I’ll update the dependency here so older versions aren’t used).