lexer freezes on this Python code

Steps to reproduce:
1. Try to run `lexer("print('\'m')")`
2. Notice that it freezes.

I traced the freeze to an infinite do-while loop in the lexer.
In the following code, input is set to the code "print('\\'m')" but current variable reaches into the billions,
currentChar === undefined, and
validator.test(currentChar) is returning false when currentChar is undefined.
```
/**
* 过滤（提取） 引号中的内容
*/
// eslint-disable-next-line
const matchQuotation = (currentChar, validator, TokenType) => {
do {
if (currentChar === '\n') {
line++;
}
currentChar = input[++current];
} while (!validator.test(currentChar));
++current;
};
```

I wonder if this has anything to do with the backslashed quotation mark. Maybe the grammar doesn't define STRING properly for escaped quotes.

I edited this issue because I first thought the bug was reproducible by parsing but it actually requires running the lexer function on the code.