Function split_by_case 

pub fn split_by_case(s: &str, separators: Option<&[char]>) -> Vec<String>

Splits a string into words at case boundaries and separators.

Handles camelCase, PascalCase, SCREAMING_SNAKE, etc. Uses separators (or STR_SPLITTERS) to split. Empty segments are preserved (e.g. foo--bar → ["foo","","bar"]).