refactor: functional component

[privatenumber]

• Since PseudoWindow is now a true functional component, it requires a parent component and is technically breaking for new Vue(PseudoWindow). However, this isn't within expected use-case and will not be handled.
• The refactor allows multiple elements to be returned (feature):

<div>
  <pseudo-window>
    <div>Hello world</div>
    <div>Hello world</div>
    <div>Hello world</div>
  </pseudo-window>
</div>