types(ts): add window declaration

Author: keithamus

index.d.ts

@@ -1,3 +1,9 @@
 export default class MarkdownToolbarElement extends HTMLElement {
   readonly field: HTMLTextAreaElement | undefined | null;
 }
+
+declare global {
+  interface Window {
+    MarkdownToolbarElement: MarkdownToolbarElement
+  }
+}