Parse supertrait auto impl syntax

- https://github.com/rust-lang/rfcs/pull/3851
- https://github.com/rust-lang/rust/issues/149556

```rust
trait Subtrait2: Supertrait2 {
auto impl Supertrait2 {
type Type1 = Self;
type Type2 = ();
}
}
```

Read RFC 3851 and Rust issue 149556 to understand the proposed syntax, then locate syn's existing trait parsing tests and entry points. Add coverage for the shown supertrait auto-impl example and verify that syn parses it successfully without regressing existing trait syntax.

Written by the indexing model from the issue text.