Function detect_return_value 

pub fn detect_return_value(
    source: &str,
    tree: &Tree,
    start_byte: usize,
    end_byte: usize,
    enclosing_fn_end_byte: Option<usize>,
    lang: LangId,
) -> ReturnKind

Detect what the extracted code range should return.

1. If there’s an explicit return statement in the range, use its expression.
2. If a variable declared in-range is referenced after the range (but within the enclosing function), that variable becomes the return value.
3. Otherwise, void.