Draft: Add #[rustfmt::sort] and implement for enum variants and struct structs

[Code0x58]

#3422: add explicit rustfmt::sort attribute which causes case sensitive sorting

• enum variants
• struct structs

This does not raise if rustfmt::sort is applied in an unhandled place (e.g. struct, member, etc.), although that might be reasonable to pursue; at a glance, rustfmt::skip does not appear to do that, which may be more reasonable given how it can reasonably apply to anything.

TODO:

• [ ] implement #[rustfmt::skip::sort]