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.

透過 SQL 解析器重現所提供的 INSERT ... SELECT ... ON CONFLICT 查詢，同時檢查 Generic 和 PostgreSQL 方言，並將其與加上括號的形式進行比較。追蹤 INSERT 陳述式解析的進入點，並確認不加括號的形式能夠被接受，同時現有案例仍然有效。

由索引模型根據 Issue 內容生成。