Function `extract_word`

#[must_use]
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.