chore: set up Copilot instructions for Rspack repository

[Copilot]

This PR adds comprehensive GitHub Copilot instructions to help the AI coding assistant understand the Rspack project structure, development workflow, and best practices.

What's Added

Created copilot-instructions.md with detailed guidance covering:

• Project Overview: Rspack as a high-performance Rust-based JavaScript bundler with webpack compatibility
• Architecture: Monorepo structure with 88 Rust crates and 179 JavaScript packages
• Development Environment: Setup requirements including Rust nightly, Node.js, and pnpm 10.10.0
• Key Commands: Build, test, lint, and format commands for both Rust and JavaScript codebases
• Code Patterns: Rust edition 2024, TypeScript with Biome linting, Jest testing framework
• Directory Structure: Detailed explanation of crates/, packages/, tests/, examples/, and other key directories
• Development Workflow: Best practices for making changes, testing, and debugging
• Plugin Development: Guidelines for both Rust and JavaScript plugin approaches
• Performance Considerations: Hot paths, memory management, and concurrency patterns
• Rstack Ecosystem: Context about related projects (Rsbuild, Rslib, Rspress, etc.)

Why This Matters

Rspack is a complex polyglot monorepo that combines Rust performance with JavaScript ecosystem compatibility. These instructions will help Copilot:

• Understand the dual Rust/JavaScript nature of the codebase
• Suggest appropriate build commands and development workflows
• Recommend patterns consistent with the project's architecture
• Provide context-aware assistance for both core Rust development and JavaScript API work

The instructions follow GitHub's best practices for Copilot coding agent configuration and provide the context needed for effective AI-assisted development on this sophisticated bundler project.

Fixes #11335.

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

@hardfist 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.

[netlify]

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ chenjiahan
❌ Copilot
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[Copilot]

Pull Request Overview

This PR sets up comprehensive GitHub Copilot instructions to help the AI assistant understand the Rspack project structure and development workflow. The instructions provide essential context about Rspack as a high-performance Rust-based JavaScript bundler with webpack compatibility.

• Adds detailed project architecture documentation covering the monorepo structure with Rust crates and JavaScript packages
• Documents development environment setup requirements and key build/test commands
• Establishes file organization guidelines and contributing best practices

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_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}