INSERT from SELECT cannot be parsed with an ON CONFLICT clause

Parsing the following SQL
```sql
INSERT INTO foo
SELECT 1, 'data'
ON CONFLICT DO NOTHING
```
fails with
```
Expected: end of statement, found: CONFLICT at Line: 3, Column: 6
```
both for Generic and PostgreSQL dialects.

However, at least in PostgreSQL it seems to be a valid construct, equivalent to
```sql
INSERT INTO foo
(SELECT 1, 'data')
ON CONFLICT DO NOTHING
```
(this one is parsed by `sqlparser` correctly).

The same query without the "ON CONFLICT" clause can be successfully parsed too.

通过 SQL 解析器复现所提供的 INSERT ... SELECT ... ON CONFLICT 查询，同时检查 Generic 和 PostgreSQL 方言，并将其与带括号的形式进行比较。跟踪 INSERT 语句解析的入口点，并确认不带括号的形式能够被接受，同时现有用例仍然有效。

由索引模型根据 Issue 内容生成。