Support to load dataset with TSV files by passing only dataset name
_The documentation is not available anymore as the PR was closed or merged._
This PR implements support to load a dataset (w/o script) containing TSV files by passing only the dataset name (no need to pass `sep='\t'`):
```python
ds = load_dataset("dataset/name")
```

The refactoring allows for future builder kwargs customizations based on file extension.

Related to #4238.