introduce divs for nodes, edges, edge labels and viewport front & back to simplify z-index ordering

packages/svelte/src/lib/components/EdgeLabel/EdgeLabel.svelte

@@ -28,7 +28,7 @@
 </script>
 
 <div
-  use:portal={'viewport'}
+  use:portal={'edge-labels'}
   class={['svelte-flow__edge-label', { transparent }, className]}
   style:cursor={selectEdgeOnClick ? 'pointer' : undefined}
   style:transform="translate(-50%, -50%) translate({x}px,{y}px)"

packages/svelte/src/lib/components/ViewportPortal/ViewportPortal.svelte

@@ -2,9 +2,9 @@
   import { portal } from '$lib/actions/portal';
   import type { ViewportPortalProps } from './types';
 
-  let { children, ...rest }: ViewportPortalProps = $props();
+  let { moveTo = 'front', children, ...rest }: ViewportPortalProps = $props();
 </script>
 
-<div use:portal={'viewport'} {...rest}>
+<div use:portal={`viewport-${moveTo}`} {...rest}>
   {@render children?.()}
 </div>

packages/svelte/src/lib/components/ViewportPortal/types.ts

@@ -2,5 +2,6 @@ import type { Snippet } from 'svelte';
 import type { HTMLAttributes } from 'svelte/elements';
 
 export type ViewportPortalProps = {
+  moveTo: 'front' | 'back';
   children?: Snippet;
 } & HTMLAttributes<HTMLDivElement>;