nodejs: regenerate `index.ts` in librarian

[JoeWang1127]

Currently, the packages/{xyz}/src/index.ts is generated by generateIndexTs, which is part of combine-library command.

We should migrate this function to librarian. This is part of the effort to eliminate gapic-node-processing.

[quirogas]

I don't think we should move this function to librarian, especially if it isn't a blocker at the moment. We should aim to own as little Node generation logic as possible to avoid hindering their generation process with our own code. For example, if they were to move to ESM, we would be responsible for updating the index.ts file. I think it would be better, to move the post-processing logic into the gapic-generator-typescript instead.

[JoeWang1127]

IMO, we should eliminate the external dependency on gapic-node-processing b/c the functions there are not generating the files from scratch, e.g., only sample and release level are updated in readme generation.

I agree that these file should be generated by node generator but we don't know when it will happen and we should not be blocked by them.