Function extract_word 

pub fn extract_word(
    line: &str,
    pos: usize,
    esc_char: Option<char>,
    is_break_char: fn(char) -> bool,
) -> (usize, &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.