Parse unnamed enum variant syntax

- https://github.com/rust-lang/rfcs/pull/3894
- https://github.com/rust-lang/rust/issues/156628

```rust
#[repr(u8)]
enum Color {
Red = 0,
Green = 1,
_ = .., // All other discriminants in `u8` are assigned to unnamed variants
}
```

Read the linked RFC PR and Rust issue first, then locate syn's enum-parsing entry point and existing enum-variant tests. Confirm that the shown #[repr(u8)] example parses, including `_ = ..` and its unnamed variants, with behavior aligned to the referenced proposal.

Written by the indexing model from the issue text.