feat: Export all `class_*` objects

[schloerke]

When working with S7 dispatch for +, I would like for the class_FOO objects to be exported. These methods do not need a public docs page (e.g. @keywords internal) but could display an example of how to make your own + method.

{GGally} would benefit having access to class_theme, class_labels, class_ggproto , etc.

Prior art: S7 package exports all class_FOO objects for us to build upon. It'd be nice to have access to all ggplot2 class_FOO objects to build upon them.

Thank you!

[teunbrand]

Would be good to get this in before the release of 4.0.0 too, just to get downstream packages more comfortable with it.

[teunbrand]

Notes from chat with Thomas:

• Export all classes that ggplot owns, including definitions for the simpler S3 classes.
  • E.g. export class_rel, but not class_gtable
• Double check that all constructors contain ... for future expansion.

[schloerke]

And a request to bump the dev version with the new exported objects. Thank you!