Add XDG Base Directory Specification support

[christopher-buss]

Summary

Implements XDG Base Directory Specification compliance for Lune's cache and state directories while maintaining full backwards compatibility with existing installations.

• Cache directory: $XDG_CACHE_HOME/lune (fallback: ~/.lune)
• State directory: $XDG_STATE_HOME/lune (fallback: ~/.lune or $XDG_CACHE_HOME/lune)
• REPL history: Stored in state directory as .lune_history
• Type definitions: Remain in ~/.lune/.typedefs/ for LSP compatibility

Key Features

• Full backwards compatibility: Automatically detects and prefers existing legacy directories
• Environment variable support: LUNE_CACHE and LUNE_STATE override all other settings
• Cross-platform: Works on Linux (XDG), Windows (Known Folders), and macOS (Standard Directories)

Implementation Details

• Uses existing directories crate
• New dirs module provides cache_dir(), state_dir(), and typedefs_dir() functions
• Integrated across all components: build cache, REPL history, LSP setup
• Automatic directory creation prevents file operation errors

Backwards Compatibility

The implementation prioritizes existing user setups:

1. If legacy .lune/target exists but XDG cache doesn't → use legacy
2. If legacy .lune_history exists but XDG state doesn't → use legacy
3. Environment variables (LUNE_CACHE/LUNE_STATE) always take precedence
4. Fallback to original logic if XDG detection fails

Test Plan

• [x] All existing functionality works unchanged
• [x] New XDG directories are created and used appropriately
• [x] Environment variable overrides work correctly
• [x] Backwards compatibility scenarios function properly
• [x] Cross-platform compatibility verified
• [x] Error handling and fallbacks tested
• [x] Thread safety and cleanup in tests

Fixes #300