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.

docs/content/en/flavor/component.md

@@ -70,6 +70,20 @@ Mount tippy on the child node instead of default tag
   <template #content>Hi!</template>
 </tippy>
 ```
+
+### Example 3
+Tippy as a child using `to="parent"`. \
+Works like `:tag="null"`, but applies the tooltip to the parent element instead of the child.
+
+```html
+<button>
+  <tippy to="parent" content-tag="div" ontent-class="content-wrapper">
+    Hi!
+  </tippy>
+  Tippy!
+</button>
+```
+
 ## Singleton
 
 ### Example 1

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

src/components/Tippy.ts

@@ -89,6 +89,7 @@ const TippyComponent = defineComponent({
   emits: ['state'],
   setup(props, { slots, emit, expose }) {
     const elem = ref<Element>()
+    const findParentHelper = ref<HTMLElement>()
     const contentElem = ref<Element>()
     const mounted = ref(false)
 
@@ -109,13 +110,24 @@ const TippyComponent = defineComponent({
     if (props.to) {
       if (typeof Element !== 'undefined' && props.to instanceof Element) {
         target = () => props.to
+      } else if (props.to === 'parent') {
+        target = () => {
+          let el = elem.value
+          if (!el) {
+            el = elem.value = findParentHelper.value!.parentElement as HTMLElement
+          }
+          return el
+        }
       } else if (typeof props.to === 'string' || props.to instanceof String) {
         target = () => document.querySelector(props.to as any)
       }
     }
 
     const tippy = useTippy(target, getOptions())
-    const contentSlot = slots.content
+    let contentSlot = slots.content
+    if (!contentSlot && props.to === 'parent') {
+      contentSlot = slots.default
+    }
 
     onMounted(() => {
       mounted.value = true
@@ -147,8 +159,6 @@ const TippyComponent = defineComponent({
     expose(exposed)
 
     return () => {
-      const slot = slots.default ? slots.default(exposed) : []
-
       const contentTag = typeof props.contentTag === 'string' ? props.contentTag as string : props.contentTag
       const content = contentSlot
         ? h(
@@ -162,6 +172,24 @@ const TippyComponent = defineComponent({
           )
         : null
 
+      if (props.to === 'parent') {
+        const result = []
+        if (!elem.value) {
+          const findParentHelperNode = h('span', {
+            ref: findParentHelper,
+            'data-v-tippy': '',
+            style: { display: 'none' },
+          })
+          result.push(findParentHelperNode)
+        }
+        if (content) {
+          result.push(content)
+        }
+
+        return result
+      }
+
+      const slot = slots.default ? slots.default(exposed) : []
       if (!props.tag) {
         const trigger = h(slot[0] as any, {
           ref: elem, 'data-v-tippy': ''