Allow "jsonl" data type signifier
### Feature request

`load_dataset` currently does not accept `jsonl` as type but only `json`. 

### Motivation

I was working with one of the `run_translation` scripts and used my own datasets (`.jsonl`) as train_dataset. But the default code did not work because

```
FileNotFoundError: Couldn't find a dataset script at jsonl\jsonl.py or any data file in the same directory. Couldn't find 'jsonl' on the Hugging Face Hub either: FileNotFoundError: Dataset 'jsonl' doesn't exist on the Hub. If the repo is private or gated, make sure to log in with `huggingface-cli login`.
```

The reason is because the script has these lines to extract the data type by its extension. Therefore, the derived type is `jsonl` which is not recognized by datasets as the error above shows.

https://github.com/huggingface/transformers/blob/ade26bf9912f69e2110137443e4406d7dbe253e7/examples/pytorch/translation/run_translation.py#L342-L356

I suppose you could argue that this is the script's fault (in which case I'll do a PR over at `transformers`) but it makes sense to me to add `jsonl` as an alias to `json` in `datasets`.


### Your contribution

At the moment I cannot work on this. I think it can be as "easy" as having an alias for json, namely jsonl.
You can use "json" instead. It doesn't work by extension names, but rather by dataset builder names, e.g. "text", "imagefolder", etc. I don't think the example in `transformers` is correct because of that Yes, I understand the reasoning but this issue is to propose that the example in transformers (while incorrect) "makes sense" in terms of user expectation. So the question is whether it would be possible to add "aliases" for common types (like "json" and "text") based on common extensions (like jsonl and txt)?