MySql dialect doesn't handle slash-escaped single quotes

The following fails to parse:

```rust
fn main() {
println!("{:?}",
sqlparser::parser::Parser::parse_sql(
&sqlparser::dialect::MySqlDialect {},
r#"INSERT INTO `example` (`col`) VALUES ('foo \'bar\' baz');"#
)
);
}
```

Instead returning:

```
Err(ParserError("Expected ), found: bar"))
```

But I believe it shouldn't — `mysqldump` generates SQL that is escaped like this, and the [docs](https://dev.mysql.com/doc/refman/8.0/en/string-literals.html) explicitly mention this as a valid escaping (see Table 9.1).

This is, according to [this doc](https://www.ibm.com/docs/en/informix-servers/12.10?topic=statements-quotation-marks-escape-characters) a difference between ANSI SQL and MySQL, but unfortunately this needs to be fixed in the tokenizer, which AFAICT the dialect system doesn't have a way to hook into?

先从 Parser::parse_sql 使用的 tokenizer 和 MySqlDialect 路径开始，然后复现给出的 INSERT 示例，以追踪带斜杠转义的单引号是如何被分词的。完成标准是：此 MySQL 输入能够成功解析，而不会将转义的引号当作字符串结尾。

由索引模型根据 Issue 内容生成。