Sibling Count with Astro

It turns out that with Astro things are fairly straight forwards

Steps:

1. Create a new Astro component. Let us call it `AstroSiblingCount`.

   In this component:

   1. First of all you need a library to parse the html, I am using linkedom
   2. grab the html content you have passed into the component in the default slot
   3. using this html I am selecting it all and creating an array of children
   4. Looping through these children I attach the --sibling-index and --sibling-child inline custom variables.

   ---
   import { parseHTML } from "linkedom";
   const html = await Astro.slots.render("default");
   const { document } = parseHTML(html);
   const [...children] = document.children;
   const siblingCount = children.length;
   ---
   {
       children?.map((htmlString, index) => {
           const siblingIndex = index + 1;
   
           return (
               <Fragment
                   set:html={`
                       <li style="--sibling-index: ${siblingIndex}; --sibling-count: ${siblingCount};">
                           ${htmlString.textContent}
                       </li>
                   `}
               />
           );
       })
   }

2. Use the component:

   <ul class="demo">
       <AstroSiblingCount>
           <li></li>
           <li></li>
           <li></li>
           <li></li>
           <li></li>
       </AstroSiblingCount>
   </ul>

   Would yield:

   <ul class="demo">
       <li style="--sibling-index: 1; --sibling-count: 5;"></li>
       <li style="--sibling-index: 2; --sibling-count: 5;></li>
       <li style="--sibling-index: 3; --sibling-count: 5;></li>
       <li style="--sibling-index: 4; --sibling-count: 5;></li>
       <li style="--sibling-index: 5; --sibling-count: 5;></li>
   </ul>