WIP: S7 theme elements

[teunbrand]

This PR aims to fix part of #6352.

In essence it converts all S3 element_*() functions and margin() to S7 classes.

As it stands, there are some backwards compatibility issues for extensions:

• Testing wether an object is a particular type element with e.g. inherits(x, "element_blank") will no longer work. From 4.3.0 onwards, using inherits(x, element_blank) will work, and S7::S7_inherits(x, element_blank) should work. Alternatively, one could in theory use inherits(x, c("element_blank", "ggplot2::element_blank")) for backward compatibility, but it'd require a change in the extension.
• Typical extraction with $, [ or [[ of properties will no longer work and extensions will have to use @. On ggplot2's end, we can provide methods for the typical generics, but we'd run into Weird S7/S3 interaction RConsortium/S7#390.
• Extensions implementing subclasses like marquee::element_marquee() or ggtext::element_markdown() will also have to write their elements as S7 classes. I cannot see a good solution to have this be backwards compatible.

[teunbrand]

A few things to consider:

• For the inherit(x, "element_blank") problem, we could consider tagging on an additional class to the S7 objects, without the ggplot2:: prefix.
• If it turns out there is too much broken in reverse dependencies, we could retain S3 classes for a cycle, implement @.element so that extensions can adapt before we commit to S7 element classes.