feat: add option to ignore unavailable entities

[hugobloem]

This pull request introduces a new feature to the stateful_scenes custom component, allowing it to ignore entities that are in an unavailable state. Additionally, it includes some code refactoring and improvements to the handling of scene states. The most important changes are:

New Feature: Ignore Unavailable Entities

• Added the ignore_unavailable configuration option to the StatefulScenes component, allowing users to specify whether entities in an unavailable state should be ignored. (custom_components/stateful_scenes/StatefulScenes.py, custom_components/stateful_scenes/config_flow.py, custom_components/stateful_scenes/const.py, custom_components/stateful_scenes/switch.py, custom_components/stateful_scenes/translations/en.json, custom_components/stateful_scenes/translations/nl.json, custom_components/stateful_scenes/translations/sk.json) [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

Code Refactoring

• Reorganized imports in StatefulScenes.py to improve readability. (custom_components/stateful_scenes/StatefulScenes.py)

Scene State Handling

• Updated the check_all_states method to correctly handle unavailable entities, ensuring that the scene is considered off if all entities are unavailable. (custom_components/stateful_scenes/StatefulScenes.py)

Switch Entity Enhancements

• Introduced the IgnoreUnavailable switch entity to manage the new ignore_unavailable feature. (custom_components/stateful_scenes/switch.py)

These changes enhance the flexibility and robustness of the stateful_scenes component, making it more resilient to entities that may become unavailable.