v0.19.0

New features:

• Effect Cells

  These can be created with .effect() on an action cell (ValueCell<void>). An effect cell
  is guaranteed to only be run once per trigger of the action cell on which the .effect() method
  is called. This is useful for running side effects, while still being able to observe the result
  of the effect as a cell.

  Example:

  final result = action.effect(() async {
      return await submitForm();
  });

• Combining Action Cells:

  Multiple action cells can now be combined into a single cell, which notifies its observers, whenever
  any of the actions trigger, using List.combined

  Example:

  final a1 = ActionCell();
  final a2 = ActionCell();
  
  final all = [a1, a2].combined;

Breaking Changes:

• Async cells created with .wait, .waitLast, .awaited now throw an PendingAsyncValueError
  instead of UninitializedCellError when the cell value is referenced while the future is pending.

  This does not affect code which uses .initialValue to handle these errors.

• Renamed onPressed and onLongPress of cell Material buttons to press and longPress,
  respectively.

  The purpose of this change, is to later allow callback based event handlers to be added using
  onPressed and onLongPress

Other Changes:

• Computed cells no longer run the computation function on initialization. Instead the first run
  of the computation function is deferred to the first time the cell value is referenced.

  NOTE: This changes slightly the semantics of computed cells, especially dynamic computed cells
  which are now dormant until their value is referenced at least once.

• Clarify semantics of ValueCell.none:

  If a null default value is given and ValueCell.none is used during the computation of the
  initial value of a cell, UninitializedCellError is thrown.

• Bug fix: Unhandled exceptions in a ValueCell.watch no longer propagate out of the watch function.