Re-work toHierarchy utility

Refactor `toHiearchy` to be more generic, performant

- Use callback to "pick" generic parent ID property instead of requiring that `pid` be hardcoded`
- Account for edge cases of an invalid parent ID
- Use Map to store node children for lookups
- Use one pass instead of removing empty nodes at the very end
- DX: use generics to type `toHierarchy`
- Make `toHierarchy` even more generic (reusable with `RecursiveList`)

Co-Authored-By: Bram <bramhavers@gmail.com>

tests/App.test.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
   import { TreeView as TreeViewNav } from "carbon-components-svelte";
   import TreeView from "./TreeView/TreeView.test.svelte";
+  import TreeViewHierarchy from "./TreeView/TreeView.hierarchy.test.svelte";
   import { onMount } from "svelte";
 
   const routes = [
@@ -9,6 +10,11 @@
       name: "TreeView",
       component: TreeView,
     },
+    {
+      path: "/treeview-hierarchy",
+      name: "TreeViewHierarchy",
+      component: TreeViewHierarchy,
+    },
   ] as const;
 
   let currentPath = window.location.pathname;