Function create_engine_with_functions_and_cache 

pub fn create_engine_with_functions_and_cache(
    grid: Grid,
    value_cache: ValueCache,
    custom_script: Option<&str>,
) -> (Engine, Option<AST>, Option<String>)

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