fix: fix global declaration

Author: calebdwilliams

Diff for: src/index.ts

@@ -1,18 +1,19 @@
 import { ElementInternals } from './element-internals';
 import { CustomStateSet } from './CustomStateSet';
 import './element-internals';
+import { IElementInternals } from './types';
 export * from './types';
 
 declare global {
   interface Window {
     CustomStateSet: typeof CustomStateSet;
     ElementInternals: typeof ElementInternals;
   }
-  interface Element {
+  interface HTMLElement {
     /**
      * Attaches an ElementInternals instance to a custom element. Calling this method
      * on a built-in element will throw an error.
      */
-    attachInternals?: () => ElementInternals
+    attachInternals(): ElementInternals&IElementInternals;
   }
 }