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.

Reproduzca la consulta INSERT ... SELECT ... ON CONFLICT proporcionada a través del analizador SQL, comprobando tanto los dialectos Generic y PostgreSQL como comparándola con la forma entre paréntesis. Siga el punto de entrada del análisis de la sentencia INSERT y confirme que se acepta la forma sin paréntesis, mientras los casos existentes siguen siendo válidos.

Escrito por el modelo de indexación a partir del texto del issue.