Function list_marker_at 

pub(crate) fn list_marker_at(
    bytes: &[u8],
    pos: usize,
) -> Option<(usize, bool, usize)>

If the line that starts at pos opens with a list marker, return Some((indent, ordered, content_start)). indent counts the leading ASCII spaces before the marker; ordered is true for \d+\. and false for - ; content_start is the byte offset of the first byte after the marker plus its trailing whitespace run. Tabs are not recognised as either indent or post-marker whitespace in MVP 0.