Add --objects option to add only object types

[TomasVotruba]

Stacked on #39 (reuses its option plumbing) - GitHub retargets it to main once #39 merges.

argtyper add-types . --objects
argtyper add-types . --literals --objects

--objects only adds object types:

• a single class, \Foo or ?\Foo
• a union made only of classes, \Foo|\Bar or \Foo|\Bar|null
• object defaults too, e.g. $status = Status::Active
• a union with a scalar, like int|\Money, is skipped

Combined with --literals, both kinds are added. The literalsOnly bool becomes apply.Options{Literals, Objects}.

On mautic: 87 types (86 object, 1 object-only union).