feat(lexer): improve validation and error handling

- Add validation for escape sequences using isValidEscapeSequence
- Add checks for empty character classes and groups
- Improve error messages for invalid quantifier modifiers
- Add validation for character range order in character classes
- Add validation for empty quantifiers and invalid ranges
- Prevent quantifiers starting with comma
- Implement validation for double modifiers (lazy/possessive)
- Add explicit handling of reserved regex symbols
- Add validation for unmatched closing brackets