v0.23.0

New features:

• Watch function keys.

  Watch functions can now be identified by keys, like cells, provided in the ValueCell.watch and
  Watch constructor.

  final w1 = ValueCell.watch(() { ... }, key: 'key1');
  final w2 = ValueCell.watch(() { ... }, key: 'key2');
  
  w1 == w2; // true
  
  // Stops both w1 and w2 since they have the same keys
  w1.stop();

• Watch functions can now be defined directly in the build method of a CellWidget without
  CellHooks:

  CellWidget.builder((_) {
    final cell = MutableCell(...);
  
    ValueCell.watch(() {
      final c = cell();
      ...
    })
    ...
  });

Breaking Changes:

• ValueCell.observe no longer returns the value of the cell. Additionally, this eliminates the need
  to catch exceptions originating from the cell when the caller is only interested in being notified
  when the value of the cell changes, but is not interested in the actual value.