fix(amplify): deploy asset Custom Resource points to TS file (#18166)

The .ts file is not packaged and exported when
distributed resulting in the CDK being unable to
find the index file. This has been corrected to
point to the .js version instead.


----

Error currently observed (CDK 1.137.0)

```
Cannot find entry file at /home/samkio/projects/activity-management-react-app/packages/server-cdk-stack/node_modules/@aws-cdk/aws-amplify/lib/asset-deployment-handler/index.ts
Subprocess exited with error 1
```

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-amplify/lib/branch.ts

@@ -223,7 +223,7 @@ class AmplifyAssetDeploymentProvider extends NestedStack {
       {
         entry: path.join(
           __dirname,
-          'asset-deployment-handler/index.ts',
+          'asset-deployment-handler/index.js',
         ),
         runtime: lambda.Runtime.NODEJS_14_X,
         handler: 'onEvent',
@@ -247,7 +247,7 @@ class AmplifyAssetDeploymentProvider extends NestedStack {
       {
         entry: path.join(
           __dirname,
-          'asset-deployment-handler/index.ts',
+          'asset-deployment-handler/index.js',
         ),
         runtime: lambda.Runtime.NODEJS_14_X,
         handler: 'isComplete',