feat(portal): support portal

src/Portal/Portal.svelte

@@ -0,0 +1,53 @@
+<script context="module">
+  /** @type {HTMLDivElement | null} */
+  let portalContainer = null;
+
+  let instances = 0;
+
+  /**
+   * Creates or returns the shared portal container
+   * @returns {HTMLDivElement}
+   */
+  function getPortalContainer() {
+    if (!portalContainer && typeof document !== "undefined") {
+      portalContainer = document.createElement("div");
+      portalContainer.setAttribute("data-portal", "");
+      document.body.appendChild(portalContainer);
+    }
+
+    return portalContainer;
+  }
+</script>
+
+<script>
+  import { onMount } from "svelte";
+
+  /** @type {null | HTMLDivElement} */
+  let portal = null;
+
+  onMount(() => {
+    instances++;
+    const container = getPortalContainer();
+
+    if (portal && container) {
+      container.appendChild(portal);
+    }
+
+    return () => {
+      instances--;
+
+      if (portal?.parentNode) {
+        portal.parentNode.removeChild(portal);
+      }
+
+      if (instances === 0 && portalContainer?.parentNode) {
+        portalContainer.parentNode.removeChild(portalContainer);
+        portalContainer = null;
+      }
+    };
+  });
+</script>
+
+<div bind:this={portal}>
+  <slot />
+</div>