Add 'val' to  VALIDATION_KEYWORDS. 
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@mariosasko not sure about how the reviewing process works. Maybe you can have a look because we discussed this elsewhere? Hi, thanks! 

Let's add one pattern with `val` to this test before merging: 
https://github.com/huggingface/datasets/blob/b88a656cf94c4ad972154371c83c1af759fde522/tests/test_data_files.py#L598 _The documentation is not available anymore as the PR was closed or merged._ @akt42 note that there is some info about splits keywords in the docs: https://huggingface.co/docs/datasets/main/en/repository_structure#split-names-keywords. I agree it's not clear that it applies not only to filenames, but to directories as well.

I think "val" should be now added to the documentation source file here: https://github.com/huggingface/datasets/blob/main/docs/source/repository_structure.mdx?plain=1#L98 @polinaeterna Thanks for notifying us that there is a list of supported keywords

I've added "val" to that list and a test.
This PR fixes #4839 by adding the word `"val"` to the `VALIDATION_KEYWORDS` so that the `load_dataset()` method with `imagefolder` (and probably, some other directives as well) reads folders named `"val"` as well.

I think the supported keywords have to be mentioned in the documentation as well, but I couldn't think of a proper place to add that.
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