Feature Request: Add Cut (Ctrl+X) and Undo (Ctrl+Z) keyboard shortcuts for text editing

## Feature Request

### Description
Currently, the Copilot CLI supports copy and paste shortcuts for text editing within the input prompt. However, **Cut (Ctrl+X)** and **Undo (Ctrl+Z)** shortcuts are not available. Adding these would make text editing in the CLI significantly faster and more intuitive.

### Current Behaviour
- ✅ Copy (Ctrl+C / Cmd+C) — works
- ✅ Paste (Ctrl+V / Cmd+V) — works
- ❌ Cut (Ctrl+X / Cmd+X) — not supported
- ❌ Undo (Ctrl+Z / Cmd+Z) — not supported

### Desired Behaviour
- **Cut**: Select text and use Ctrl+X (or Cmd+X on macOS) to cut it to clipboard
- **Undo**: Use Ctrl+Z (or Cmd+Z on macOS) to undo the last text edit action

### Use Case
When composing or editing longer prompts in the CLI, users often need to rearrange text or revert accidental edits. Without cut and undo, users have to manually retype or re-enter their input, which slows down the workflow.

### Priority
Good to have — not a blocker, but a quality-of-life improvement for users who prefer typing and editing directly in the CLI rather than composing prompts externally.

### Additional Context
Standard terminal emulators and most CLI tools with line editors (e.g., readline-based tools) support these shortcuts. Aligning with these conventions would make the Copilot CLI feel more natural to use.