Add comprehensive GitHub Copilot instructions for foresta-js repository

[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 foresta-js codebase.

The instructions cover all essential development workflows:

Bootstrap & Build Process:

• Exact commands for dependency installation (npm ci ~2-3 seconds, npm install ~45 seconds)
• Build process using Rollup (npm run build <1 second)
• All timing expectations with "NEVER CANCEL" warnings included

Testing & Validation:

• Jest test suite execution (npm test <1 second, 17 tests)
• Coverage reporting (npm run test:coverage <2 seconds, 92%+ coverage)
• Manual validation scenarios with expected outputs from node examples/basic.js

Development Workflow:

• Watch mode for continuous testing (npm run test:watch)
• Linting with ESLint using proper environment flags
• CI validation steps matching the GitHub Actions workflow

Key Information:

• Complete file structure and important locations
• All 5 selector types supported by the library (#identifier, ExpressionType, *, property modifiers, path queries)
• Dependencies (esprima for runtime, jest/rollup/eslint for development)
• Expected build outputs and warnings

Validation:
Every command in the instructions has been thoroughly tested by following them exactly from a clean repository state. The instructions are designed to be copy-pastable and work reliably every time, eliminating guesswork for coding agents working with this JavaScript AST selector engine library.

Fixes #5.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.