feat(portal): add Portal component

[metonym]

Related #2150

Closes #309, closes #835

Cherry-picks only Portal-related changes, omitting FloatingPortal for now.

Portal would immediately unblock Modal usage by consumers. FloatingPortal is better suited for internal usage with tooltips/popovers etc..

This adds a new Portal component that renders its content into a fixed container appended to document.body. This enables components like modals, tooltips, and dropdowns to escape parent overflow constraints and z-index stacking contexts, which is essential for proper layering and positioning in complex UI layouts.

Changes

• Add Portal component that renders content into a fixed container in document.body
  • Implement a shared container approach with automatic cleanup when all instances unmount

Usage

<script>
  import { Portal } from "carbon-components-svelte";
</script>

<div>
  <Portal>
    Parent is document.body, not div
  </Portal>
</div>

[metonym]

Released in v0.95.0