Add `Portal` component

[metonym]

A fairly basic component for a component library.

A Portal would be immediately useful for modal, among other use cases. Eventually, all "popout" components (e.g., Modal, Tooltip, menus) should be portalled by default.

This would support: https://github.com/carbon-design-system/carbon-components-svelte/issues?q=is%3Aissue%20state%3Aopen%20label%3Aportal

[brunnerh]

I have found that I generally do not need portals at all.

Often the better alternative is to use the client-side API to mount a new component instance on the document.body, especially for modal dialogs which then can be used with an async code flow like:

// this function mounts a component and handles events for cancel/submit
const result = await showSelectThingDialog();
if (result == null)
    return;

// do something with result

This also makes sure that the dialog always is shown with a clean state.

Playground example

Whether this makes sense for things like dropdowns within components is questionable, though. It also would potentially introduce Svelte 5 compatibility issues, since components are mounted/unmounted differently there.