Add self-hosting documentation to README

[Copilot]

This PR adds comprehensive documentation for users who want to build and run the GitHub Copilot Chat extension from source code instead of installing it from the VS Code marketplace.

What's Added

A new "Self-Hosting / Building from Source" section that covers:

• System Requirements: Node.js 22.x+, npm 9.0.0+, Python 3.10-3.12, Git LFS, and Windows build tools
• Installation Steps: Repository cloning, dependency installation, and optional token setup
• Build Process: Using npm run build to compile the extension
• Packaging: Creating .vsix files with npm run package and installing them locally in VS Code
• Development Mode: Watch mode with npm run watch and VS Code debugging configuration
• Configuration Notes: Requirements for GitHub Copilot subscription and authentication

Why This Is Needed

Previously, the README only focused on end-user features and marketplace installation. Users who wanted to:

• Build from source for development
• Create custom builds
• Self-host the extension
• Contribute to the project

Had to dig through the CONTRIBUTING.md file or figure out the build process themselves. This addition makes the self-hosting process accessible directly from the main README while keeping it concise and pointing to CONTRIBUTING.md for detailed development information.

Validation

All documented commands, scripts, and configurations have been verified to exist in the repository:

• ✅ npm scripts (build, package, watch, get_token)
• ✅ VS Code launch configurations ("Launch Copilot Extension")
• ✅ Build tools (esbuild, vsce)
• ✅ Setup scripts and requirements

The documentation provides clear, step-by-step instructions that users can follow to successfully build and install the extension locally.

Created from VS Code via the GitHub Pull Request extension.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.