Add comprehensive GitHub Copilot instructions for source-map-parser development

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed instructions for working effectively in the source-map-parser codebase.

What's included

The instructions cover the complete development workflow for this Rust workspace project:

Build & Test Process:

• Exact commands with validated timeouts (e.g., cargo build --workspace takes 60 seconds, wasm-pack test --node takes 4.5 minutes)
• Critical "NEVER CANCEL" warnings for long-running operations
• Both native Rust and WASM build/test workflows

WASM Development:

• Complete wasm-pack installation and setup process
• Node.js WASM package build and testing
• Troubleshooting for common network issues (binaryen download failures)

Code Quality:

• Formatting (cargo fmt) and linting (cargo clippy) workflows
• Acceptable vs. must-fix warning categories

Validation Scenarios:

• Functional test examples for core source map parsing features
• Node.js WASM integration verification
• Stack trace parsing validation

Repository Structure:

• Key directories and file organization
• Core API overview for both Rust library and WASM bindings
• Common development commands reference

Validation

All commands and examples in the instructions have been thoroughly tested and validated:

• Build times measured and documented with appropriate timeout buffers
• WASM functionality confirmed working with real Node.js examples
• Alternative workflows provided for network-restricted environments
• Functional scenarios verified to work correctly

The instructions follow the imperative tone requirement and provide exhaustive detail to ensure any developer can successfully work with this codebase from a fresh clone.

Fixes #5.

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