title: how to list all available slots in vue component
body: <p>I'm working on tool for design system and manual isolated testing of Vue components.</p>
  
  <p>I would like to list all available slots and scopedSlots without rendering a component in DOM. Ideally - to do it on client side, but also possible on server-side (using vue test utils or whatever that works).</p>
  
  <p>I tried to access slots from <code>vm</code> but <code>vm.$slots</code> has only slots that filled from parent component.</p>
  
  <p>Is there a way to get list of all slots somehow? I would like to avoid writing our own parser for templates... </p>