nodejs: evaluate `src/index.ts` in keep list

The `src/index.ts` in libraries are handwritten. We use this issue to track such cases and document the reason why they can't be generated.

- google-cloud-aiplatform, added via https://github.com/googleapis/google-cloud-node/pull/8666
- google-cloud-vision, added via https://github.com/googleapis/google-cloud-node/pull/8748
- google-cloud-translate, via https://github.com/googleapis/google-cloud-node/pull/8749

To unblock GAPIC library migrating to librarian, we add these `src/index.ts` in the keep list.

Adding `src/index.ts` to keep means this file will not be regenerated during library generation and the Node team would have to maintain `src/index.ts` manually, such as adding or removing imports.

For example, if google-cloud-aiplatform creates a new API version, the Node team would have to add the new version to the `src/index.ts` file.

Start with the librarian keep-list configuration and inspect the handwritten src/index.ts files for google-cloud-aiplatform, google-cloud-vision, and google-cloud-translate. Read pull requests #8666, #8748, and #8749 for context. Done means the three files are kept from regeneration and their manual-maintenance reasons are documented.

Written by the indexing model from the issue text.