refactor: 💡 rename required_by_dataset_viewer: enables_preview
Change the response of `/valid`, from:

```json
{
  "valid": ["datasets", "that", "have", "/first-rows"]
}
```

to

```json
{
  "preview": ["datasets", "that", "only", "have", "/first-rows"],
  "viewer": ["datasets", "that", "have", "/rows", "and", "/size"],
  "valid": ["preview", "and", "viewer"]
}
```

Note that `valid` is the same as before, and will be removed in a future PR, once the Hub has been updated to use "preview" and "viewer".

### Notes

I propose to first try this. If it works, we will remove the `valid` field, update the docs (https://huggingface.co/docs/datasets-server/valid) and do the same switch for `/is-valid`.

refactor: 💡 rename required_by_dataset_viewer: enables_preview: Change the response of `/valid`, from:

```json
{
  "valid": ["datasets", "that", "have", "/first-rows"]
}
```

to

```json
{
  "preview": ["datasets", "that", "only", "have", "/first-rows"],
  "viewer": ["datasets", "that", "have", "/rows", "and", "/size"],
  "valid": ["preview", "and", "viewer"]
}
```

Note that `valid` is the same as before, and will be removed in a future PR, once the Hub has been updated to use "preview" and "viewer".

### Notes

I propose to first try this. If it works, we will remove the `valid` field, update the docs (https://huggingface.co/docs/datasets-server/valid) and do the same switch for `/is-valid`.
