Exports a new constant `ALL_SPEC_FEATURES` containing all available jsii extension features. This allows consumers of the `@jsii/spec` package to programmatically access the complete list of features without having to maintain their own copy.

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By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

…cript implementation (#5027)

In recent changes, we dropped as many methods from `Jsii$Proxy` as possible. This includes the implementation of `toString()` from `Jsii$Proxy`, which means that `toString()` falls back to the Java implementation instead of the JavaScript one.

```java
@jsii(module = $Module.class, fqn = "aws-cdk-lib.IResolvable")
@Proxy(IResolvable.Jsii$Proxy.class)
public interface IResolvable extends JsiiSerializable {
  List<String> getCreationStack();
  ResolutionTypeHint getTypeHint();
  Object resolve(@NotNull IResolveContext paramIResolveContext);
  
  String toString();   // <---- ⚠️ note this!
  
  public static interface Jsii$Default extends IResolvable {
    @NotNull
    default List<String> getCreationStack() {
       // ... 
    }
    
    @nullable
    default ResolutionTypeHint getTypeHint() {
       // ... 
    }
    
    @NotNull
    default Object resolve(@NotNull IResolveContext context) {
       // ... 
    }

    // <--- ❌ but missing here!
  }
}
```

This doesn't fail because only there is there also a default `toString()` implementation on all objects in Java.

There was no problem when we called `toString()` on a proxy of a public JSII class, but in case we were using a pure interface proxy (when the concrete class was private or even anonymous), we wouldn't forward to the JavaScript-side `toString()` but call the default Java implementation instead.

Fix that and add a test for this behavior.

---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0