Add support for <Tippy> as a child (issue #123 in 6.x)

Having <Tippy> as a child of an element to tooltip was a nice use-case
in 4.x. See https://github.com/KABBOUCHI/vue-tippy/issues/123.
We re-add this in a similar manner.

Using to="parent" (as a fixed string), one can now use either the main
<Tippy> slot ("default") or its specific "content" slot to specify
the tooltip content.
The tooltip will be triggered on the parent element without
adding a specifc Tippy node to the DOM.

This should provides a non-interfering way to specify tooltips.

Implementation:
For the to="parent" case, we need to find the parent DOM element, but in
the "worst" case <Tippy> won't render any DOM nodes at all, which could
help us to find the parent.
Instead, we initially render a hidden <span> (and the tooltip content).
Using the span as a template ref ('findParentHelper'), we can get the
parent and assign that to the actual 'elem' ref. This will trigger a
re-render, where we can drop the helper <span> again.

playground/pages/Index.vue

@@ -88,6 +88,33 @@
       </tippy>
     </div>
 
+    <div class="mt-6">
+      <span class="font-semibold mr-4">Tippy as a child component with prop content:</span>
+
+      <button class="text-sm py-2 px-3 bg-gray-900 text-white rounded-lg">
+        Hi
+        <tippy to="parent" content="Test"></tippy>
+      </button>
+    </div>
+
+    <div class="mt-6">
+      <span class="font-semibold mr-4">Tippy as a child component with #content template:</span>
+
+      <button class="text-sm py-2 px-3 bg-gray-900 text-white rounded-lg">
+        Hi
+        <tippy to="parent"><template #content>Test</template></tippy>
+      </button>
+    </div>
+
+    <div class="mt-6">
+      <span class="font-semibold mr-4">Tippy as a child component with direct content:</span>
+      
+      <button class="text-sm py-2 px-3 bg-gray-900 text-white rounded-lg">
+        Hi
+        <tippy to="parent">{{counter}} and counting</tippy>
+      </button>
+    </div>
+
     <div class="mt-6">
       <span class="font-semibold mr-4">useTippy + callbacks(console.log):</span>
       <button