Clarify the specification for synthetic and fresh names.

[RCHowell]

Description

The PartiQL Specification uses “_1" and “_n” in some interesting ways. In the absence of some explicit bind name, "synthetic" or "fresh" names are used. It's not clear if this is implementation defined or there are some rules to how these names are enumerated.
Ask

This behavior should either explicitly defined or clearly designated as implementation defined.

Discussion

https://github.com/orgs/partiql/discussions/84