✨ Add Jsonl corpus support

[AnotherStranger]

To be removed, once it is done: Please add the appropriate label to this ticket, e.g. feature or enhancement.

Is your feature/enhancement request related to a problem? Please describe.
For annotation of my texts, I mostly use doccano. Doccano also offers Jsonl as export format of the annotated datasets. I find this format very handy because it is easy to read and easy to process. It would be very convenient for me if flair supported Jsonl corpora.

Describe the solution you'd like
I have already implemented a JsonlCorpus for NER and text classification and would be willing to provide a PR if there is broader interest in a JsonlCorpus.

Additional context
Let me know if you are interested.