aws-cdk-lib: CustomResourceProvider does not allow .mjs ECMAScript modules

### Describe the feature

When creating a CustomResourceProvider with NODEJS_20_X runtime and specifying a codeDirectory that contains an index.mjs file, the deployment fails. ("Error: cannot find codeDirectory/index.js").

### Use Case

Since it is the default now to have an index.mjs ECMAScript module when you create a node 20.x Lambda Function in the AWS console, I would expect that this is possible for CDK Custom Resources as well.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.151.0

### Environment details (OS name and version, etc.)

MacOS Sonoma

Start at the CustomResourceProvider implementation and trace how codeDirectory is resolved for the NODEJS_20_X runtime. Reproduce the deployment case with a directory containing index.mjs and compare it with the current index.js expectation. Done means a CustomResourceProvider using that runtime can deploy an index.mjs module without the reported cannot-find error, with relevant tests updated.

Written by the indexing model from the issue text.