refactor: Rename Deno main file to index.ts and update import

deno/main_test.ts

@@ -1,5 +1,5 @@
 import { assertEquals } from "@std/assert";
-import { add } from "./main.ts";
+import { add } from "./index.ts";
 
 Deno.test(function addTest() {
   assertEquals(add(2, 3), 5);