Enhancement: Add structured error types for better diagnostics

[clank-hayen]

Problem

Currently, all compiler errors are plain strings:

// typechecker/typechecker.hpp
struct TypeError {
    std::string message;
    int line;
    std::string filename;
};

// Usage
error(state, "method '" + mcall.method_name + "' not found on struct '" + obj_type.base_type + "'", mcall.line);

This makes it difficult to:

• Provide structured error codes for tooling (LSP, IDEs)
• Offer specific fix suggestions
• Test for specific error conditions
• Localize error messages

Suggested Solution

Introduce error categories and structured data:

enum class ErrorCode {
    E001_UNDEFINED_VARIABLE,
    E002_UNDEFINED_FUNCTION,
    E003_TYPE_MISMATCH,
    E004_ARGUMENT_COUNT_MISMATCH,
    E005_UNDEFINED_METHOD,
    // ...
};

struct TypeError {
    ErrorCode code;
    int line;
    string filename;
    
    // Structured data for the specific error type
    variant<
        UndefinedSymbol,      // { name: string }
        TypeMismatch,         // { expected: string, got: string }
        ArgCountMismatch,     // { expected: int, got: int }
        // ...
    > details;
    
    string format() const;  // Generate human-readable message
};

Benefits

1. Tooling integration: Error codes can be used by LSP for quick fixes
2. Testing: Can test for specific error types, not just message substrings
3. Localization: Error templates can be translated
4. Consistency: Structured data ensures all necessary context is captured

Implementation Notes

• Start with type errors, extend to parser errors later
• Keep backward compatibility by implementing `format()` method
• Consider adding source ranges (not just line numbers)

Priority

Medium - Improves developer experience significantly.