Support <tippy> as a child

[kkuegler]

This PR add the "tippy component as a child" behavior from the 4.x version back.

It is slightly different from the old functionality, because this now requires the to='parent' prop to be set on the Tippy compoent. This should make it easier to understand, where this tooltip belongs and also simplifies the implementation.

I like this "Tippy as a child" style, because in my mind a tooltip is just a small aspect of a component. "Hiding" it as a child seems nicer to me than having Tippy be the parent of the tooltipped element. This also behaves like :tag="null", i.e. not adding new DOM nodes under or around the tooltipped element. This helps with adding a tooltip without breaking existing styles.

There is some more technical details in the commit messages.

This change allows the following new ways of using Tippy:

<button>
  Click me!
  <tippy to="parent">
    I'm the tooltip
  </tippy>
</button>

or

<button>
  Click me!
  <tippy to="parent" content="I'm the tooltip"/>    
</button>

or

<button>
  Click me!
  <tippy to="parent">
    <template #content>
      I'm the tooltip
    </tooltip>
  </tippy>
</button>

[nuxt-studio[bot]]

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[mikoca]

Nice I like it.