GH-3098/add fewnerd corpus

[whoisjones]

closes gh-3098.

adds the fewnerd corpus, ready for review. @alanakbik

can be called like this:

  corpus = FEWNERD()
  corpus = FEWNERD("intra")
  corpus = FEWNERD("inter")
  corpus.make_label_dictionary("ner")

[alanakbik]

The black checks are failing, can you do black --config pyproject.toml flair/ && isort flair/ and black --config pyproject.toml tests/ && isort tests/ in the project root?

[whoisjones]

depends on gh-3090. was required for running black correctly.

[alanakbik]

@whoisjones thanks for adding this!