Support generating PR descriptions based on a template

[kycutler]

Closes microsoft/vscode-pull-request-github#5683 along with microsoft/vscode-pull-request-github#8025

[Copilot]

Pull Request Overview

This PR adds support for generating pull request descriptions based on a provided template. It allows users to specify a template that the AI will follow when generating PR descriptions, ensuring consistency with project standards and requirements.

Key Changes

• Added template parameter to the prompt system for PR description generation
• Modified parsing logic to handle template-based descriptions differently
• Enhanced user prompt to include template instructions when provided

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/extension/prompts/node/github/pullRequestDescriptionPrompt.tsx	Adds template parameter to prompt interfaces and includes template instructions in the system rules and user message
src/extension/prompt/node/githubPullRequestTitleAndDescriptionGenerator.ts	Updates generator to accept and process template parameter, modifies parsing logic to handle template-based responses

[alexr00]

I haven't forgotten about this PR, I just see that the stests are failing and I'm trying to understand why.

[alexr00]

Since the prompt changed, you'll also need to run npm run simulate. See https://github.com/microsoft/vscode-copilot-chat/blob/6fa866f39d84179d004edab249b9b1c57260cb89/CONTRIBUTING.md.