feat(ngrx-toolkit): add rollback and nested dot-path key support to withUndoRedo

[asollberger]

• Add rollback(savepoint) method that restores state to a Date.now() timestamp, moving intermediate changes to the redo stack
• Support dot-path keys (e.g. 'filter.searchTerm') in the keys option, enabling tracking of nested state properties
• Add type-safe DotPath utility type for compile-time validation of nested key paths
• Add tests for rollback, nested dot-path keys, and complex nested state
• Add rollback section to documentation
• Add demo component showcasing undo/redo with rollback and savepoints