Function unescape_string 

pub fn unescape_string(s: &str) -> Result<String, String>

Unescape special characters in a quoted string.

Per TOON spec §7.1, only these escape sequences are valid:

• \\ → \
• \" → "
• \n → newline
• \r → carriage return
• \t → tab

Any other escape sequence MUST cause an error.

Errors

Returns an error if the string contains an invalid escape sequence or if a backslash appears at the end of the string.