Carry exact expected statement type in `ParserError`

Currently `sqlparser-rs` return a `&str` to tell which type of token parser is expecting:
``` rust
fn expected(&self, expected: &str, found: Token) -> Result
```

When a SQL string is not completed.
Return an exact `ExpectType` will be better, this can deliver help for developing tools like SQL complete tools etc.
``` rust
fn expected(&self, expected: Vec, found: Token) -> Result
```

首先定位 ParserError 的实现以及 issue 中描述的预期函数，然后检查不完整 SQL 的解析方式。定义 ExpectType 值必须表示的内容，并验证 SQL 补全工具能够保留精确的预期结果，同时覆盖不完整语句。

由索引模型根据 Issue 内容生成。