Function to_snake_case 

pub fn to_snake_case(s: &str) -> String

Converts a string from any common case to snake_case.

Word boundaries are detected at:

• Non-alphanumeric characters (spaces, hyphens, underscores, colons, etc.)
• Transitions from lowercase or digit to uppercase (camelCase -> camel_case)
• Within consecutive uppercase letters, before the last if followed by lowercase (XMLParser -> xml_parser)