Function plan_add_parameter 

pub async fn plan_add_parameter(
    ctx: &RefactoringContext,
    def_rel_path: &str,
    function_name: &str,
    param_name: &str,
    param_type: Option<&str>,
    default_value: &str,
    position: Option<usize>,
) -> Result<AddParameterOutcome, String>

Build an add-parameter plan without touching the filesystem.

file is the path to the file containing the function definition. function_name is the name of the function to modify. param_name is the name of the new parameter. param_type is the optional type annotation (for typed languages). default_value is the value to insert at call sites. position is the 0-based index to insert at; None means last.