Can't parse jsonb extractions in an on conflict in Postgres

The parser does not support the following:

```sql
INSERT INTO table_with_constraint_over_jsonb (a_number, a_jsonb, a_string)
VALUES ($1, $2, $3)
ON CONFLICT (a_number, (a_jsonb->>'name'))
DO UPDATE SET a_string = EXCLUDED.a_string
RETURNING *;
```

It errors on the `(a_jsonb->>'name')` part with the error: `ParserError("Expected: identifier, found: ( at Line: 1, Column: 119")`.

Start by reproducing the parser error with the SQL statement in the issue, focusing on the parenthesized jsonb extraction in the ON CONFLICT target. Trace the SQL parser's handling of ON CONFLICT expressions, then verify that this statement parses successfully without regressing other conflict targets.

Written by the indexing model from the issue text.