Initialize SDK as early as possible

[limbonaut]

Currently, we initialize at MODULE_INITIALIZATION_LEVEL_SCENE, which is not the earliest level. This is because core singletons are unavailable until this stage in the current stable Godot release.

A PR has already been merged that makes core singletons available at INITIALIZATION_LEVEL_CORE:

• Register Engine, OS, ProjectSettings, and Time singletons in time for INITIZATION_LEVEL_CORE godotengine/godot#98862](Register Engine, OS, ProjectSettings, and Time singletons in time for INITIZATION_LEVEL_CORE godotengine/godot#98862

These changes are planned for version 4.4, which is expected to be released sometime next year. If we raise the minimum compatibility to 4.4, we’ll be able to initialize earlier. In the meantime, we might be able to use preprocessor checks as a workaround.

Related Godot issues:

• Singletons such as RenderingServer should be accessible in MODULE_INITIALIZATION_LEVEL_SCENE godotengine/godot-cpp#1180
• 4.0 GDExtension: global_get_singleton not returning expected singletons. godotengine/godot#64975