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

[filiso]

• 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.

[kharvd]

any reason to limit to 999 messages?

Suggested change

	COMMAND_BACK_REGEX = r":b(?:ack)? (\d{1,3})"
	COMMAND_BACK_REGEX = r":b(?:ack)? (\d+)"

[filiso]

Good catch, no there is no reason to keep this limit.