Ability to customize keyboard shortcut for `/voice` dictation toggle

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

First off, I want to say that the recently added /voice feature is incredibly useful and a fantastic addition to the GitHub Copilot CLI. It has significantly streamlined my workflow.

However, I would like to propose a feature to make the user experience even better: customizable keyboard shortcuts for dictation toggling.

Currently, the /voice mode instructions state: "Voice ready. Hold space to record, or ctrl+x v to toggle dictation." While this works, the Ctrl+x v sequence is not particularly convenient or ergonomic for frequent use. Allowing users to remap this toggle to a shortcut of their choosing would greatly improve accessibility and efficiency.

### Proposed solution

Add a configuration option in `~.copilot/settings.json` that allows users to define a custom keybinding to toggle dictation mode, overriding the default Ctrl+x v.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by tracing the /voice command and the existing Ctrl+x v dictation toggle, then inspect how ~/.copilot/settings.json configuration is loaded and how keyboard bindings are represented. The change is done when users can set a custom dictation-toggle shortcut in settings.json and the configured shortcut overrides the default while the existing behavior remains available.

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