Snowflake delcare variable parsing fails

The following fails when parsing with the Snowflake dialect. [Cross-referenced from the C# implementation](https://github.com/TylerBrinks/SqlParser-cs/issues/14).

```
let sql = concat!(
"DECLARE \
query_id_1 VARCHAR; \
query_id_2 VARCHAR; \
BEGIN \
SELECT TOP 100 LIVINGUNITID AS L1 \
,PERSONID AS P1 \
FROM ASSET_VIEWS.CV.CV_PII_ADV_CROSSWALK; \
query_id_1 := SQLID; \
RETURN [query_id_1]; \
END;");

snowflake().verified_stmt(sql);
```

首先使用 snowflake().verified_stmt(sql) 运行 Snowflake 方言复现，重点关注所示的 DECLARE、BEGIN/END、变量赋值和 RETURN 语法。跟踪解析器失败，并为此语句添加覆盖；当提供的 SQL 能够在 Snowflake 方言下成功解析时即表示完成。

由索引模型根据 Issue 内容生成。