fix(mobx): include observableKind and debugObjectName in observe events of boxed values - #4761

observe on a boxed observable, an observable object property or an observable map entry delivers update events without observableKind and debugObjectName, although IValueDidChange declares both as required and the docs list them for every observe event. The fireImmediately event in observe already includes them, so a listener sees them on the first call and undefined on every later one:

const box = observable.box(1, { name: "box" })
observe(box, c => console.log(c.observableKind, c.debugObjectName), true)
box.set(2)
// before: "value box", then "undefined undefined"
// after:  "value box", then "value box"

The fix adds the two fields to the change object in ObservableValue.setNewValue_. The object is only built when a listener is registered, so nothing changes otherwise.

Code change checklist

• Added/updated unit tests
• Updated /docs. For new functionality, at least API.md should be updated (not needed, the docs already describe this behaviour)
• Verified that there is no significant performance drop (npm -w mobx run test:performance) (the script is a shell script and I couldn't run it on Windows; the change adds two properties to an object that is only created when there are listeners)