Improvement: WestCommandsType should be a class

See https://github.com/zephyrproject-rtos/west/pull/920#discussion_r3367818364

Actually, would it be simpler and clearer to just promote WestCommandsType to some very basic but actual Python class?

I did consider this, and gave it a brief try, but it felt like a much more invasive refactoring than I was comfortable doing, and that would further explode the size of this #920 PR (which as you said has evolved quite a bit). It most likely should have been a class from the very start tbh - _west_commands_list feels like a constructor, _west_commands_maybe_delist feels like a string representation, _west_commands_merge would be an elegant iadd....

Make sure to include the _west_commands_manifest_dirs in the class somehow - it could be that a subclass of dict would be most suitable for WestCommandsType.