Acceptance confirmation for CLI vi/vim mode feature (issue #13)

### Describe the feature or problem you'd like to solve

Acceptance confirmation for vi/vim input mode in Copilot CLI (issue #13)

### Proposed solution

Acceptance confirmation for the addition of a Vim mode to the Copilot CLI, allowing modal editing (Normal/Insert/Visual) with standard Vim keybindings for efficient navigation and editing. Next steps: detail specific behaviors to support, keyboard shortcuts, and mode transitions. Identify any external libraries or dependencies needed.

### Example prompts or workflows

- Users can switch between Normal, Insert, and Visual modes within CLI input.
- Standard Vim navigation/editing keys (h, j, k, l, w, d, y, etc.) are supported.
- Efficient editing of suggested commands or input prompts.

### Additional context

Accepted based on user request and alignment with power-user workflow improvements. See https://github.com/github/copilot-cli/issues/13 for original discussion.