Text builder with custom separator line boundaries
Gently pinging @lhoestq Hi ! Interresting :)

Could you give more details on what kind of separators you would like to use instead ? In my case, I just want to use `\n` but not `U+2028`. Ok I see, maybe there can be a `sep` parameter to allow users to specify what line/paragraph separator they'd like to use Related to:
- #3729 
- #3910 Thanks for requesting this enhancement. We have recently found a somehow related issue with another dataset:
- #3704

Let me make a PR proposal.
**Is your feature request related to a problem? Please describe.**
The current [Text](https://github.com/huggingface/datasets/blob/207be676bffe9d164740a41a883af6125edef135/src/datasets/packaged_modules/text/text.py#L23) builder implementation splits texts with `splitlines()` which splits the text on several line boundaries. Not all of them are always wanted. 

**Describe the solution you'd like**
```python
                if self.config.sample_by == "line":
                    batch_idx = 0
                    while True:
                        batch = f.read(self.config.chunksize)
                        if not batch:
                            break
                        batch += f.readline()  # finish current line
                        if self.config.custom_newline is None: 
                            batch = batch.splitlines(keepends=self.config.keep_linebreaks)
                        else:
                            batch = batch.split(self.config.custom_newline)[:-1]
                        pa_table = pa.Table.from_arrays([pa.array(batch)], schema=schema)
                        # Uncomment for debugging (will print the Arrow table size and elements)
                        # logger.warning(f"pa_table: {pa_table} num rows: {pa_table.num_rows}")
                        # logger.warning('\n'.join(str(pa_table.slice(i, 1).to_pydict()) for i in range(pa_table.num_rows)))
                        yield (file_idx, batch_idx), pa_table
                        batch_idx += 1
```

**A clear and concise description of what you want to happen.**
Creating the dataset rows with a subset of the `splitlines()` line boundaries.