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_

先追蹤 /voice 命令和現有的 Ctrl+x v 聽寫切換，然後檢查如何載入 ~/.copilot/settings.json 設定，以及鍵盤繫結是如何表示的。當使用者可以在 settings.json 中設定自訂的聽寫切換快速鍵，且設定的快速鍵會覆寫預設快速鍵，同時現有行為仍然可用時，此變更就完成了。

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