Function parse_timeline 

pub fn parse_timeline(
    body: &str,
    section_start_line: usize,
    event_names: &HashSet<&str>,
    errors: &mut Vec<ParseError>,
) -> Vec<Rel>

Parse the ## Timeline section body into preceded_by relationships.

Each non-blank line is a chain: Event A -> Event B -> Event C. event_names are entity names from the ## Events section.