Fix static bootstrap method in template polyfill externs.

Author: bicknellr

packages/template/externs/template.d.ts

@@ -11,6 +11,10 @@
  */
 
 // eslint-disable-next-line no-var
-declare var HTMLTemplateElement: {
-  bootstrap(): void;
-};
+declare global {
+  interface HTMLTemplateElementConstructor {
+    bootstrap(): void;
+  }
+}
+
+export {};