Function invoke_step 

pub async fn invoke_step(
    actx: &AsyncContext,
    idx: usize,
    params: &[JsArg],
    data_table: Option<&[Vec<String>]>,
    doc_string: Option<&str>,
    source: &str,
) -> Result<StepOutcome, ScriptError>

Invoke step idx with cucumber-extracted args, the optional data table and doc string, against the current World. A thrown JS error becomes a ScriptError carrying the .js location.