feat: Add /iterate command

[yozerpp]

/iterate sends the prompt with all files by dividing files to batches that fit into the output and context limits.

for example converting multiple files to another language in one prompt, use /add src/main/java and /iterate convert these files into C#..

Aider will divide files into batches that fit the output and context limit of the model (Output tokens that will be produced for a file are assumed to be more than the token count of the file for the worst case scenerio. Limits are evaluated from model metadatas.) and send each batch with the prompt. /iterate does this by resetting the context to the state before firing of the /iterate command after execution of each batch.

Also added tests for following conditions:
- Every request retains the chat history until the /iterate command but not the history of other iterations.
- Added files and history until the /iterate is unmodified.
- Every file is processed(even if a single file that'll be sent with the request exceeds the limits.) and no duplicate processing

p.s. sorry for mess about the PRs