TsInterface should not have exported static members

[vegegoku]

For example the following java class

@TsInterface
@TsName(namespace = "dh")
public class HasEventHandling {

    @JsProperty(namespace = "dh.Client")
    public static final String EVENT_REQUEST_FAILED = "requestfailed";
    
}

will produce the following TS

export namespace dh {

   export interface HasEventHandling {

      get EVENT_REQUEST_FAILED():string;

   }
 
}

we need to error out for TsInterface static members.

[niloc132]

Note that this might be related to the attempt to export the property to a different type, which we've agreed should be forbidden (at least for now).