Update index.d.ts

Use the right way to add custom element to TypeScript definitions.

Sadly, I didn't find any official docs on this matter, only some posts/repos:
- https://justinfagnani.com/2019/11/01/how-to-publish-web-components-to-npm/
- https://github.com/runem/lit-analyzer/tree/master/packages/lit-analyzer#-no-unknown-tag-name

Author: mgenware

index.d.ts

@@ -4,7 +4,7 @@ export default class ImageCropElement extends HTMLElement {
 }
 
 declare global {
-  interface Window {
-    ImageCropElement: typeof ImageCropElement
+  interface HTMLElementTagNameMap {
+    'image-crop': ImageCropElement
   }
 }