Interactivity API: Move init.js to TypeScript (#61723)

* Move init.js to typescript

* Throw an error for regions without parent

Co-authored-by: DAreRodz <darerodz@git.wordpress.org>
Co-authored-by: sirreal <jonsurrell@git.wordpress.org>
Co-authored-by: michalczaplinski <czapla@git.wordpress.org>

packages/interactivity/src/init.js → packages/interactivity/src/init.ts

@@ -1,7 +1,7 @@
 /**
  * External dependencies
  */
-import { hydrate } from 'preact';
+import { hydrate, type ContainerNode, type ComponentChild } from 'preact';
 /**
  * Internal dependencies
  */
@@ -11,7 +11,10 @@ import { directivePrefix } from './constants';
 
 // Keep the same root fragment for each interactive region node.
 const regionRootFragments = new WeakMap();
-export const getRegionRootFragment = ( region ) => {
+export const getRegionRootFragment = ( region: Element ): ContainerNode => {
+	if ( ! region.parentElement ) {
+		throw Error( 'The passed region should be an element with a parent.' );
+	}
 	if ( ! regionRootFragments.has( region ) ) {
 		regionRootFragments.set(
 			region,
@@ -29,7 +32,7 @@ function yieldToMain() {
 }
 
 // Initial vDOM regions associated with its DOM element.
-export const initialVdom = new WeakMap();
+export const initialVdom = new WeakMap< Element, ComponentChild[] >();
 
 // Initialize the router with the initial DOM.
 export const init = async () => {