Create copilot-instructions.md (#18490)

Author: T-Gro

.github/copilot-instructions.md

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+# GitHub Copilot Instructions for F# Compiler
+
+## Language and Environment
+- Use modern F# with functional-first approach
+- Target .NET Standard 2.0 for compatibility
+- Avoid external dependencies - the codebase is self-contained
+
+## Code Style and Standards
+- Follow [coding standards](docs/coding-standards.md), especially:
+  - Prefer immutability and pure functions
+  - Use consistent naming (see abbreviation guide)
+  - Process complex types recursively with pattern matching
+  - Avoid single-character identifiers except in established patterns
+- This project uses .fsi signature files. When adding a new public API to a namespace/module which will be consumed from other file, add it to the respective .fsi signature file as well
+- Follow [overview](docs/overview.md) for key compiler data formats and representations
+
+## Type System Handling
+- When working with `TType` instances, use appropriate stripping functions:
+  - `stripTyparEqns` - Removes inference equations from type parameters
+  - `stripTyEqns` - Removes type equations and type abbreviations
+  - `stripTyEqnsAndErase` - Also erases compilation representations
+  - Match the appropriate stripper to the context (e.g., display vs compilation)
+  - Check equality with `typeEquiv` after stripping rather than direct comparison
+
+## Core Data Structures
+- Use F# discriminated unions for type representations
+- Respect representation hiding through access control
+- Use `remapX` functions when transforming types across boundaries
+- Be aware of performance implications for recursive type operations
+
+## Documentation and Naming
+- Document public APIs with XML comments
+- Use descriptive function names that indicate transformation direction
+- Follow established naming conventions in error messages