Implement a :back command to go back to a specific message pair.

- Implements functionality to go back to a specific message. - The footer, which currently includes the conversation cost and the number of tokens, will now display the number (identifier) of each message-pair. - A message pair is a user's message followed by the model's response. - Typing ':b' or ':back' will undo the last message-pair, including the user's message unlike ':r' which keeps it. - Typing ':b X' or ':back X' will go back in the conversation state to message-pair X and confirm it via displaying a string in the console. - Going back to X means that X is still part of the conversation with later messages removed.