Define "has no body" to mean `;`, and "has a body" to mean either block body or expression body

We have some interest in simplifying the language so that `get;` or `void M();` is referred to as "not having a body," and thus we can say "has a body" instead of repeatedly saying "has a block body or an expression body" as a pair.

One thing to determine is whether the grammar currently refers to the `;` as a body.

Original discussion:
- https://github.com/dotnet/csharpstandard/pull/681#discussion_r2384262560
- https://github.com/dotnet/csharpstandard/pull/681#discussion_r2384264834