Updated the behaviour of commands:
- `lmcli edit`
- by default create a new branch/message branch with the edited contents
- add --in-place to avoid creating a branch
- no longer delete messages after the edited message
- only do the edit, don't fetch a new response
- `lmcli retry`
- create a new branch rather than replacing old messages
- add --offset to change where to retry from
- Split pkg/cli/cmd.go into new pkg/cmd package
- Split pkg/cli/functions.go into pkg/lmcli/tools package
- Refactor pkg/cli/openai.go to pkg/lmcli/provider/openai
Other changes:
- Made models configurable
- Slight config reorganization
