Resolve data_files by split name

As discussed in https://github.com/huggingface/datasets/issues/3027 we should automatically infer what file is supposed to go to what split automatically, based on filenames.

I added the support for different kinds of patterns, for both dataset repositories and local directories:

```
    Input structure:

        my_dataset_repository/
        ├── README.md
        └── dataset.csv

    Output patterns:

        {"train": ["*"]}
```
```
    Input structure:

        my_dataset_repository/
        ├── README.md
        ├── train.csv
        └── test.csv

        my_dataset_repository/
        ├── README.md
        └── data/
            ├── train.csv
            └── test.csv

        my_dataset_repository/
        ├── README.md
        ├── train_0.csv
        ├── train_1.csv
        ├── train_2.csv
        ├── train_3.csv
        ├── test_0.csv
        └── test_1.csv

    Output patterns:

        {"train": ["*train*"], "test": ["*test*"]}
```
```
    Input structure:

        my_dataset_repository/
        ├── README.md
        └── data/
            ├── train/
            │   ├── shard_0.csv
            │   ├── shard_1.csv
            │   ├── shard_2.csv
            │   └── shard_3.csv
            └── test/
                ├── shard_0.csv
                └── shard_1.csv

    Output patterns:

        {"train": ["*train*/*", "*train*/**/*"], "test": ["*test*/*", "*test*/**/*"]}
```

and also this pattern that allows to have custom split names, and that is the structure used by #3098 for `push_to_hub` (cc @LysandreJik ):

```
    Input structure:

        my_dataset_repository/
        ├── README.md
        └── data/
            ├── train-00000-of-00003.csv
            ├── train-00001-of-00003.csv
            ├── train-00002-of-00003.csv
            ├── test-00000-of-00001.csv
            ├── random-00000-of-00003.csv
            ├── random-00001-of-00003.csv
            └── random-00002-of-00003.csv

    Output patterns:

        {
            "train": ["data/train-[0-9][0-9][0-9][0-9][0-9]-of-[0-9][0-9][0-9][0-9][0-9].*"],
            "test": ["data/test-[0-9][0-9][0-9][0-9][0-9]-of-[0-9][0-9][0-9][0-9][0-9].*"],
            "random": ["data/random-[0-9][0-9][0-9][0-9][0-9]-of-[0-9][0-9][0-9][0-9][0-9].*"],
        }
```

You can check the documentation about structuring your repository [here](https://52640-250213286-gh.circle-artifacts.com/0/docs/_build/html/repository_structure.html). cc @stevhliu 


Fix https://github.com/huggingface/datasets/issues/3027
Fix https://github.com/huggingface/datasets/issues/3212

In the future we can also add support for dataset configurations.

Really cool!
When splitting by folder, what do we use for validation set ("valid", "validation" or both)?

> When splitting by folder, what do we use for validation set ("valid", "validation" or both)?

Both are fine :) As soon as it has "valid" in it

Merging for now, if you have comments about the documentation we can address them in subsequent PRs :)

Thanks for the comments @stevhliu :) I just opened https://github.com/huggingface/datasets/pull/3233 to take them into account