Function extract_word

pub fn extract_word<'l>(
    line: &'l str,
    pos: usize,
    esc_char: Option<char>,
    break_chars: &BTreeSet<char>,
) -> (usize, &'l str)

Given a line and a cursor position, try to find backward the start of a word. Return (0, line[..pos]) if no break char has been found. Return the word and its start position (idx, line[idx..pos]) otherwise.