Updated the behaviour of commands:
- `lmcli edit` - no longer deletes messages past the edited message
- `lmcli retry` - creates a branch from the previous user message
- 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
