Idea: Support for some form of "portal-slots"

[LinusBorg]

<portal to="destination>
  <div slot="header">
    this should go to the header slot
  </div>
</portal>

<portal to="destination>
  <div slot="footer">
    This should go tto the footer slot
  </div>
</portal>

<portal-target :slots=['header', 'default', 'footer']/>

This is similar to #26 but I think both APIs would serve different purposes. While #26 would enable an arbitrary number of portals to send to one portal-target with some position prop (to ensure order), The suggestion above would enable declaratively "sorting" a finite number of inputs by leveraging the normal slot functionality.

This features could also be combined I think - in the wormhole, we save all incoming seperated by slot and then position.

[LinusBorg]

This will not be a separate feature but will be achievable ina way with the upcoming PortalTargetGroup