Rewrite the selection change observer in `src/annotator/selections` to
make it easier to understand how this functionality works and change it
in future.

The major change is to remove the dependency on the `Observable` class
provided by `zen-observable`, which adds cognitive overhead here, isn't used
elsewhere and is stuck on an old version due to subtle breaking changes in newer releases.

Replace it instead with a `SelectionObserver` class which is modeled
after DOM APIs like `MutationObserver`. It takes a callback in the
constructor that is invoked with the selected `Range` or `null` when the
selection changes and provides a `disconnect` method which stops
watching for future changes. The implementation only uses DOM APIs.