Add comprehensive GitHub Copilot instructions for development workflow

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot with detailed instructions for working effectively in the unplugin-vue-router codebase.

What's included

The instructions cover all essential development workflows with validated commands and timing:

• Build and setup: Complete dependency installation with pnpm, build processes using tsdown
• Testing: Unit tests with Vitest, type checking, and coverage reporting
• Validation: Playground-based testing scenarios for route generation and functionality
• Development tools: Linting with Prettier, documentation building with VitePress
• Repository structure: Key directories, file organization, and important files to check after changes

Key features

• All commands validated: Every command mentioned has been tested with actual timing measurements
• Timeout guidance: Specific timing expectations with "NEVER CANCEL" warnings for long-running operations
• Functional validation: Step-by-step scenarios for testing file-based routing functionality in the playground
• CI requirements: Exact commands that must pass for the GitHub Actions workflow to succeed

Example validation workflow

The instructions guide developers through complete testing scenarios:

1. Make changes to route generation logic
2. Run pnpm run build to rebuild the library
3. Start playground with pnpm run play
4. Test dynamic routes like /users/[id] and verify TypeScript completions
5. Run pnpm run lint and pnpm run vitest before committing

This ensures that GitHub Copilot can provide accurate guidance for both new contributors and experienced developers working on this Vue Router plugin.

Fixes #694.

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[cloudflare-workers-and-pages]

Deploying unplugin-vue-router with    Cloudflare Pages

Latest commit:	f7e0d20
Status:	✅  Deploy successful!
Preview URL:	https://a02c0265.unplugin-vue-router.pages.dev
Branch Preview URL:	https://copilot-fix-694.unplugin-vue-router.pages.dev

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