(appsync): Appsync with NodejsFunction lambda resolver doesn't work with ESM format

### What is the problem?

I have an Appsync API with a lambda resolver in nodejs, created with the NodejsFunction function. When changing the bundling option to `format: OutputFormat.ESM` I see the following error in the Appsync logs:
`"error": {
"message": "index.main is undefined or not exported",
"type": "Runtime.HandlerNotFound"
},`

When I remove the format option, everything is working fine again.

### Reproduction Steps

1. Create a GraphqlApi
2. Create a lambda function with NodejsFunction and set the bundling option `format: OutputFormat.ESM`
3. Add the lambda function as a resolver.

### What did you expect to happen?

Appsync executes the queries without any errors just like when the format option wasn't set

### What actually happened?

Got the following error:

`"error": {
"message": "index.main is undefined or not exported",
"type": "Runtime.HandlerNotFound"
},`

### CDK CLI Version

2.15.0 (build 151055e)

### Framework Version

_No response_

### Node.js Version

14

### OS

macOS 12.1

### Language

Typescript

### Language Version

_No response_

### Other information

_No response_

Start by reproducing the GraphqlApi and NodejsFunction setup with bundling format OutputFormat.ESM on Node.js 14, then inspect the generated handler and the AppSync error. Done means AppSync executes resolver queries without the Runtime.HandlerNotFound error.

Written by the indexing model from the issue text.