[heycam/webidl] ObservableArray types in namespace (#1000)

per https://heycam.github.io/webidl/#idl-observable-array,

> Observable array types must only be used as the type of regular attributes.

and readonly regular attributes can be a namespace member, so it seems
```idl
namespace A {
  readonly attribute ObservableArray<boolean> foo;
};
```
is allowed. 
 
But in https://heycam.github.io/webidl/#dfn-attribute-getter, we check if the target is interface
> 2. if target is an interface, and attribute is a regular attribute:
>    ....
>    4. If attribute’s type is an observable array type, then return esValue’s backing observable array exotic object for attribute.

which makes ObservableArray type in namespace doesn't work.

cc @domenic 

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