Only-data dataset loaded unexpectedly as validation split
I see two options:
1. drop the "dev" keyword since it can be considered too generic
2. improve the pattern to something more reasonable, e.g. asking for a separator before and after "dev"
```python
["*[ ._-]dev[ ._-]*", "dev[ ._-]*"]
```

I think 2. is nice. If we agree on this one we can even decide to require the separation for the other split keywords "train", "test" etc. Yes, I had something like that on mind: "dev" not being part of a word.
```
"[^a-zA-Z]dev[^a-zA-Z]" Is there a reason why we want that regex? It feels like something that'll still be an issue for some weird case. "my_dataset_dev" doesn't match your regex, "my_dataset_validation" doesn't either ... Why not always "train" unless specified? The regex is needed as part of our effort to make datasets configurable without code. In particular we define some generic dataset repository structures that users can follow

> ```
> "[^a-zA-Z]*dev[^a-zA-Z]*"
> ```

unfortunately our glob doesn't support "^": 

https://github.com/fsspec/filesystem_spec/blob/3e739db7e53f5b408319dcc9d11e92bc1f938902/fsspec/spec.py#L465-L479 > "my_dataset_dev" doesn't match your regex, "my_dataset_validation" doesn't either ... Why not always "train" unless specified?

And `my_dataset_dev.foo` would match the pattern, and we also have the same pattern but for the "validation" keyword so `my_dataset_validation.foo` would work too > The regex is needed as part of our effort to make datasets configurable without code

This feels like coding with the filename ^^' This is still much easier than having to write a full dataset script right ? :p
## Describe the bug
As reported by @thomasw21 and @lhoestq, a dataset containing only a data file whose name matches the pattern `*dev*` will be returned as VALIDATION split, even if this is not the desired behavior, e.g. a file named `datosdevision.jsonl.gz`.