Prompt template support

### Feature summary

Add support to use prompt templates

### What problem are you trying to solve?

Prompt templates can be defined in VS Code and reused through chats. It would be great to have this feature

### Proposed solution

_No response_

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

Start by reviewing how VS Code defines and reuses prompt templates, then trace how chats are handled in the GitHub Copilot app. Clarify the supported template format, invocation flow, and expected chat behavior before implementation. Done means prompt templates can be reused through chats as described in the issue.

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