Function find_prev_word_start 

pub fn find_prev_word_start(text: &Rope, cursor_byte: usize) -> usize

Finds the start offset of the previous word (or punctuation token) moving backward from cursor_byte.

Behavior matches classic editor Ctrl + Left:

1. If preceded by whitespace (non-newline), skips backward across the whitespace.
2. If preceded by a newline, stops at that line boundary.
3. Identifies whether the preceding token is a word or punctuation sequence.
4. Moves backward across consecutive characters of that same kind.
5. Returns the starting byte offset of that token.