Function create_engine_with_functions_and_spill 

pub fn create_engine_with_functions_and_spill(
    grid: Arc<Grid>,
    spill_map: Arc<SpillMap>,
    computed_map: Arc<ComputedMap>,
    custom_script: Option<&str>,
) -> (Engine, Option<AST>, Option<String>)

Create a Rhai engine with built-ins, custom functions, and shared maps. Returns the engine, compiled AST (if any), and any error message.