Misleading error message for typo in IF NOT EXISTS

See also https://github.com/GreptimeTeam/greptimedb/issues/3471.

For sqlparser-rs -

```rust
fn main() {
let sql = "CREATE TABLE IF NOT EXIST ();";

let dialect = GenericDialect {};

let ast = Parser::parse_sql(&dialect, sql).unwrap();

println!("AST: {ast:?}");
}
```

throws:

```
called `Result::unwrap()` on an `Err` value: ParserError("Expected end of statement, found: NOT at Line: 1, Column 17")
```

IIUC if we found `IF` already, the next keywords SHOULD BE `NOT EXIST` and thus the parsing logics should be `.expect_keywords`.

从提供的 Rust 示例开始，使用 GenericDialect 和 Parser::parse_sql 解析 CREATE TABLE IF NOT EXIST 语句，然后跟踪 CREATE TABLE 和 IF NOT EXISTS 的 parser 路径。完成的标准是，该拼写错误会产生适当的 parser 错误，而不是误导性的语句结束消息。

由索引模型根据 Issue 内容生成。