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
```

先定位 ParserError 的實作以及 issue 中描述的預期函式，然後檢查不完整 SQL 的解析方式。定義 ExpectType 值必須代表的內容，並驗證 SQL 補全工具能保留精確的預期結果，同時涵蓋不完整的陳述式。

由索引模型根據 Issue 內容生成。