Add support for attribute lookup for multi slot transclusion

Currently ti use multi slot transfusion, we need to name our elements using not standard HTML elements. Like in a snippet below.

<!-- parent.html -->
<div data-my-directive>
  <my-slot>
    <span>Lorem Ipsum</span>
  </my-slot>
</div>

This is not HTML5 compliant.
Enabling attribute lookup, may help this. Consider following syntax:

<!-- parent.html -->
<div data-my-directive>
  <div data-my-slot>
    <span>Lorem Ipsum</span>
  </div>
</div>

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