Build system cleanup + dataloading scaffolding + JSON dataloading removed

[Hop311]

• Moved openvic-dataloader submodule to openvic-simulation.
• Modified SConstruct to build openvic-simulation as a library using its own SConstruct, similar to what openvic-simulation does to openvic-dataloader and openvic-dataloader does to lexy.
• Moved extension sources into openvic-extension subdir.
• Removed JSON based "custom mode" loading (compatibility mode is the current focus).
• Added Logger::warning (alongside Logger::info and Logger::error).
• Moved "hardcoded defines" (mapmodes, building types) to simulation submodule.
• Added religion mapmode.
• Added prototype gdscript-level AssetManager which queries the C++ layer to lookup asset-paths and then loads and caches them, making them available to UI nodes.
• Added methods to pass a list of root directories to the C++ layer, those being the base install directory + an ordered list of mod directories. Currently this has no UI, the base directory must be passed as a command line argument after --compatibility-mode or edited in GameStart.gd, and any mod directories also need to be edited in GameStart.gd.

Requirement commented:

• MAP-34
• UI-24, UI-124, UI-125, UI-127, UI-563
• UIFUN-142

[BrickPi]

works as expected, tested on Win64; code lgtm.