Add comprehensive test suite and GitHub Actions CI

[will-laverty]

Summary

Adds proper test coverage and CI that runs on every PR.

Test Suite (30+ tests)

LuaExecutionTests:

• Initialization & cleanup (including double-cleanup safety)
• Script execution: simple returns, multi-value, no-return, all types (string, int, float, bool, nil)
• Error handling: syntax errors, runtime errors, undefined functions, file not found
• File execution: valid scripts, syntax errors, missing files
• Stack management: size tracking, withUnchangedStack
• Global variables: set/get for all types, undefined globals, script access
• Function calls: with params, string params, no params, no return, multi-return, undefined function errors
• Function registration: Swift → Lua callback
• Boolean correctness: validates the -1 → 1 fix from PR Fix critical bugs and add proper error handling #1
• Complex scripts: fibonacci, string manipulation, math operations
• Multiple independent Lua instances
• LuaError description coverage

LuaMacroTests:

• Simple function expansion
• Different function names
• Empty function body
• Diagnostic on struct, class, and variable (validates the new diagnostic from PR Fix critical bugs and add proper error handling #1)

GitHub Actions CI

• Runs on: push to main + all PRs to main
• macOS 14 runner with Xcode 15.4
• Installs Lua via Homebrew
• Builds and runs full test suite
• Concurrency grouping to cancel stale runs