feat: Deprecate Shanten.number_characters and Shanten.number_isolated_tiles

[Apricot-S]

This PR deprecates the public attributes Shanten.number_characters and Shanten.number_isolated_tiles.

• Both attributes were previously exposed as public fields but actually represent internal state used during backtracking.

• Accessing them now triggers a DeprecationWarning with the message:

  "`<name>` is deprecated. This attribute reflects internal state and should not be used."

• No setter is provided; attempts to assign to these attributes will raise AttributeError.

• In addition, following the proposal in Shanten.number_characters and Shanten.number_isolated_tiles Have Inappropriate Names #57, the internal fields have been renamed to more accurate identifiers (_flag_four_copies and _flag_isolated_tiles) to avoid confusion with tile categories or counts.

Benefits of this change

• Improves API clarity
• Prevents misuse of unstable internal values
• Aligns the implementation with the naming improvements discussed in Shanten.number_characters and Shanten.number_isolated_tiles Have Inappropriate Names #57

fix #57

[Nihisil]

Thank you for this fix