pub fn one_line_len(&self) -> Option<usize> {
        if self.start.line == self.end.line {
            Some(self.end.column + 1 - self.start.column)
        } else {
            None
        }
    }
If this one resembles a single line, returns the a `Some(len)` value.
For multilines this cannot account for the length.
doc-chunks/src/span.rs