Rename frontend.tsx file to frontend.ts

The file doesn't use any jsx syntax.
[MAILPOET-3920]

assets/js/src/marketing_optin_block/frontend.ts

@@ -1,7 +1,6 @@
 /**
  * External dependencies
  */
-/* eslint-disable react/react-in-jsx-scope */
 import { registerCheckoutBlock } from '@woocommerce/blocks-checkout';
 
 /**

webpack.config.js

@@ -448,7 +448,7 @@ const marketingOptinBlock = Object.assign({}, wpScriptConfig, {
     'marketing-optin-block-frontend': path.resolve(
       process.cwd(),
       'assets/js/src/marketing_optin_block',
-      'frontend.tsx'
+      'frontend.ts'
     ),
   },
   output: {