wit: Propagate feature gate annotations to inner items

I think that these two snippets should be equivalent:

@since(version = 0.1.0)
interface handler {
  handle: func();

  @since(version = 0.2.0);
  new-handle: func();
}

@since(version = 0.1.0)
interface new-interface {
  @since(version = 0.1.0)
  hello: func();

  @since(version = 0.2.0);
  new-handle: func();
}

The more explicit version would be the canonical form reflected in the parse tree.

I think this goes beyond just authoring convenience. When every item is annotated it becomes more difficult to pick out the exceptional items; see e.g. https://github.com/WebAssembly/wasi-http/blob/main/wit/types.wit

Start by locating the WIT parser and its parse-tree representation, then compare how interface-level and item-level @since annotations are recorded. Use the two snippets in the issue as the behavioral example; done means the implicit annotations are represented explicitly and the exceptional newer item remains distinguishable.