Function insert_at 

pub fn insert_at(
    text: &str,
    start: usize,
    end: usize,
    insert: &str,
) -> (String, usize)

text with insert put in place of the selection from start to end, and where the caret lands after it. Positions count UTF-16 code units, as the browser counts them; a backwards selection is the same selection, and a position past the end is the end.