This PR implements support for `ON CONFLICT` clause chain, e.g.
```
INSERT INTO ct(id, x, y) VALUES (4, 'x', 'y1'), (5, 'a1', 'b'), (3, '_', '_')
ON CONFLICT(x) DO UPDATE SET x = excluded.x || '-' || x, y = excluded.y || '@' || y, z = 'x'
ON CONFLICT(y) DO UPDATE SET x = excluded.x || '+' || x, y = excluded.y || '!' || y, z = 'y'
ON CONFLICT DO UPDATE SET x = excluded.x || '#' || x, y = excluded.y || '%' || y, z = 'fallback';
```
Reviewed-by: Jussi Saurio <jussi.saurio@gmail.com>
Reviewed-by: Preston Thorpe <preston@turso.tech>
Closes#3453
6fd2ad2f5e