Get placeholder value type.
export type PlaceholderValue<T> = T extends IPlaceholder<infer A> ? A : T extends IComposedPlaceholder<infer B, any, any> ? B : T;
If the type is not a placeholder, return the same type.
IPlaceholder\<IAttribute\> is resolved as IAttribute IPlaceholder\<IAttribute\> is resolved as IAttribute IComposedPlaceholder\<IMeasure\> is resolved as IMeasure null is resolved as null