[LUA] Add custom Lua require loader to resolve modules via the engine file system.

[Neloreck]

Notes

• Extends the gamedata modules root feature ([FEATURE] Add gamedata as lua modules root. #1610), which relied on package.path and could only resolve loose files — scripts packed inside game archives were not visible to require.
• The loader is registered at package.loaders[2]: after package.preload, before the default package.path loader. On a miss it returns a candidate-path message string instead of raising, so the remaining loaders still run and require error output stays informative.
• Priority follows the engine file system: loose gamedata files override archived ones (archives are registered first during fsgame.ltx scan, and later registrations win), consistently with how every other game asset is resolved.
• Loaded chunks are named @<resolved path>, so script stack traces point to real file paths.

Changes

• Adds gamedata_module_loader — a package.loaders entry that maps require("scripts.folder.file") to gamedata\scripts\folder\file.script and loads it via FS.r_open/luaL_loadbuffer, supporting both loose and archived files.
• Adds setup_gamedata_module_loading, grouping loader registration and the existing package.path gamedata append into a single setup step.
• Replaces the inline package.path block in CScriptEngine::init with a setup_gamedata_module_loading call.