Use internal Luau headers with compatibility layer for luaL_register

[stayaway245]

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PR Review: Use internal Luau headers with compatibility layer for luaL_register

This PR has merged changes to use internal Luau headers directly with a compatibility layer for luaL_register functionality. The changes look well-implemented and address compatibility issues between Lua and Luau APIs.

Key Changes

1. Build Configuration:

   • Modified .github/workflows/build.yml to use internal Luau headers from the project
   • Removed steps to install external Lua or create custom headers
   • Set environment variables to properly locate internal headers
   • Created CMake finder modules for Lua and LuaFileSystem that point to internal headers

2. Compatibility Layer:

   • Added a compatibility layer in source/lfs.c that implements luaL_register using Luau's native API
   • Created custom function luau_register that mimics the behavior of Lua's luaL_register
   • Added function remapping for lua_pushcfunction to use Luau's lua_pushcclosurek

Benefits

• Simplifies the build process by removing external Lua dependencies
• Makes the codebase more consistent by using the internal Luau headers everywhere
• Maintains compatibility with code that expects traditional Lua C API functions

The implementation is clean and focused on addressing the specific compatibility issue while making minimal changes to the existing code.