Support `excluded` table alias in `INSERT ... ON CONFLICT`

The `ON CONFLICT` clause of the `INSERT` statement implicitly provides a table alias named `excluded` that represents the row proposed for insertion.

Example:
```sql
INSERT INTO distributors (did, dname)
VALUES (5, 'Gizmo Transglobal'), (6, 'Associated Computing, Inc')
ON CONFLICT (did) DO UPDATE SET dname = EXCLUDED.dname;
```

[PostgreSQL Docs](https://www.postgresql.org/docs/current/sql-insert.html)

Found while testing DoltHub postgres schema.