Show `@Metadata` diffs

[Goooler]

When a class is compiled with a new Kotlin version (e.g., 2.3.0 → 2.4.0) without code changes, Diffuse shows ∆ in the report but no details. We can add support for @Metadata diffs with two constraints to minimize noise:

1. Omit data1 and data2 due to their large size, comparing only remaining fields (e.g., mv, k).
2. Only show @Metadata diffs when a class has changed without any method or field additions/removals.

[JakeWharton]

I'm open to showing the metadata version change, the same way we'd show the java classfile version change.

I'm a bit skeptical about anything else in the metadata since it's mostly a hack to carry additional information that can't go anywhere else in the Java classfile format.