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>

types/TreeView/TreeView.svelte.d.ts

@@ -15,7 +15,7 @@ type $RestProps = SvelteHTMLElements["ul"];
 
 type $Props = {
   /**
-   * Provide a nested array of nodes to render
+   * Provide an array of nodes to render
    * @default []
    */
   nodes?: Array<TreeNode>;
@@ -94,11 +94,6 @@ export default class TreeView extends SvelteComponentTyped<
    */
   collapseAll: () => void;
 
-  /**
-   * Create a nested array from a flat array
-   */
-  toHierarchy: (flatArray: TreeNode[] & { pid?: any }[]) => TreeNode[];
-
   /**
    * Programmatically expand a subset of nodes.
    * Expands all nodes if no argument is provided