Option to disable artificial streaming delay in terminal output

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

The CLI adds an artificial delay when rendering streamed output, producing a typewriter-like effect. For users who read completed blocks rather than watching tokens appear, this slows down the perceived response time without adding value. There is no setting to disable or reduce this behaviour.

### Proposed solution

Add a setting (e.g. in settings.json or via a `--no-animate` flag) to render streamed output as fast as it arrives from the API, without an artificial per-token or per-chunk delay. This would benefit users on fast connections who prefer to read complete output, and would improve the experience when using screen readers or piping output to other tools.

### Example prompts or workflows

1. settings.json: `{ "streamingDelay": 0 }` or `{ "instantOutput": true }`
2. CLI flag: `copilot --no-animate`
3. A `/speed` or `/animation` slash command to toggle during a session

### Additional context

Running on Linux (Debian via WSL2) with a wired connection. The delay is most noticeable on longer responses where waiting for the full render adds significant wall-clock time compared to the actual API response time.