Add support for `.conll` file format in datasets

### Feature request

I’d like to request native support in the Hugging Face datasets library for reading .conll files (CoNLL format). This format is widely used in NLP tasks, especially for Named Entity Recognition (NER), POS tagging, and other token classification problems.

Right now `.conll` datasets need to be manually parsed or preprocessed before being loaded into datasets. Having built in support would save time and make workflows smoother for researchers and practitioners.

I propose - 
Add a conll dataset builder or file parser to datasets that can:
- Read `.conll` files with customizable delimiters (space, tab).
- Handle sentence/document boundaries (typically indicated by empty lines).
- Support common CoNLL variants (e.g., CoNLL-2000 chunking, CoNLL-2003 NER).
- Output a dataset where each example contains:
- tokens: list of strings
- tags (or similar): list of labels aligned with tokens

Given a .conll snippet like:
```
EU NNP B-ORG
rejects VBZ O
German JJ B-MISC
call NN O
. . O
```


The dataset should load as:
```
{
  "tokens": ["EU", "rejects", "German", "call", "."],
  "tags": ["B-ORG", "O", "B-MISC", "O", "O"]
}
```




### Motivation

- CoNLL files are a standard benchmark format in NLP (e.g., CoNLL-2003, CoNLL-2000).
- Many users train NER or sequence labeling models (like BERT for token classification) directly on `.conll` 
- Right now you have to write your own parsing scripts. Built in support would unify this process and would be much more convenient 

### Your contribution

I’d be happy to contribute by implementing this feature. My plan is to- 
- Add a new dataset script (conll.py) to handle .conll files.
- Implement parsing logic that supports sentence/document boundaries and token-label alignment.
- Write unit tests with small `.conll` examples to ensure correctness.
- Add documentation and usage examples so new users can easily load `.conll` datasets.

This would be my first open source contribution, so I’ll follow the `CONTRIBUTING.md` guidelines closely and adjust based on feedback from the maintainers.

Labels: name: enhancement

That would be cool ! feel free to ping me if I can help reviewing a PR

hey @namesarnav  , are you still planning to PR this? happy to coordinate or take it on otherwise. looks like a clean
  packaged_modules/conll/ builder following the csv/json/text pattern.

 @lhoestq , thanks for the pre-review offer. if @namesarnav has moved on, i can pick this up and ping you for review. 

hey @CrypticCortex , ngl i got too busy with uni and i actually forgot about this haha, if you'd like we can work together on it. 
this feature is something i really needed at the time. 

hey @namesarnav glad you're back. happy to pair....was thinking i scaffold the packaged_modules/conll/ builder (mirroring csv/text shape) and you weigh in on the column-mapping defaults since you've worked with these formats more. but lmk if you want to split it differently or just stay in the review loop.


@CrypticCortex yeah sure, go ahead. use text as base since that is more relevant to regular conll files, lmk when you get that done! 

@namesarnav done — pr at [#8219](https://github.com/huggingface/datasets/pull/8219) . text-builder shape worked well as you suggested. supports both .conll and .conllu extensions out of the box, with the conll-u variant pre-applying comment_prefix="#". 14 tests cover conll-2003 / 2000 / u + edge cases (docstart, tail sentence, tab delimiter, pad/truncate). lmk if you want any of the defaults tweaked.

@lhoestq ready for review whenever you have a minute.

@CrypticCortex reviewed the pr and it all looks good, great job!