matchOperator method
Implementation
String? matchOperator() => switch (_op) {
TokenKind.EQUALS => '=',
TokenKind.INCLUDES => '~=',
TokenKind.DASH_MATCH => '|=',
TokenKind.PREFIX_MATCH => '^=',
TokenKind.SUFFIX_MATCH => '\$=',
TokenKind.SUBSTRING_MATCH => '*=',
TokenKind.NO_MATCH => '',
_ => null
};