identifier method
void
identifier()
Implementation
void identifier() {
while (peek() != null && isAlphaNumeric(peek()!)) {
advance();
}
String text = source.substring(start, current);
TokenType type = keywords[text] ?? TokenType.identifier;
if (type == TokenType.boolean) {
addToken(type, text == 'true');
} else {
addToken(type);
}
}