Add API for getting the value of an observation

[chasenlehara]

Right now you can use can-reflect to get the value of an observation, but it’d be nice if there was a more simple API for doing that. Related, some observables build on top of can-observation and make it possible to set values; a consistent, friendly API for both getting and setting values would be nice.

Two options stick out to me:

1. Document get() and suggest that other observables implement a set() method. That would look something like:

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

const observation = new Observation(() => 15);
observation.get(); // -> 15

const observable = keyObservable(/* ... */);
observable.set(22);
observable.get(); // -> 22

2. Create a configurable value property on observations:

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

const observation = new Observation(() => 15);
observation.value; // -> 15

const observable = keyObservable(/* ... */);
observable.value = 22;
observable.value; // -> 22

[chasenlehara]

I’m going to go with option 2, creating a value property on observations.