* Move reaction methods of IUserMessage to IReactionMessage
Moves the reaction-related methods contained in IUserMessage to the IReactionMessage type. Updates the type of ISystemMessage so that it implements IReactionMessage.
* Move rest reaction implementation to RestReactionMessage
Copies the reaction implementation from RestUserMessage to RestReactionMessage. Updates RestUserMessage and RestSystemMessage to be derived from RestReactionMessage instead of RestMessage.
* Move WS reaction implementation to SocketReactionMessage
Copies the reaction implementation from SocketUserMessage into SocketReactionMessage. Updates SocketSystemMessage and SocketUserMessage to use SocketReactionMessage as the base class.
* docs: update summary for ReactionMessage classes
* Remove ReactionMessage types, move reaction impl to IMessage
Removes the IReactionMessage and derived types, which was unnecessary since all classes derived from IReactionMessage were IMessage.
Moves the reaction implementation to IMessage and derived types.
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