[main] Support UNION typing in the main branch

[alancai98]

UNION is already in the v1 plan and plan typer. For adding UNION support in Scribe partiql/partiql-scribe#55, we may need to cut a release off of main/v0.14.6 branch that includes UNION in the plan and plan typer (assuming we aren't releasing v1 soon).

UNION in the main branch does not support set quantifiers. To align the main branch plan with v1, we would need to update the plan, which may necessitate a major version release.

Another complication is whether the removal of null and missing should be included in such a release, given the usage of these unknown types in Scribe.

[alancai98]

Closed by #1506 and is part of the v0.14.6 (and later) releases.