New: Americas NLI dataset
This PR adds the [Americas NLI](https://arxiv.org/abs/2104.08726) dataset, extension of XNLI to 10 low-resource indigenous languages spoken in the Americas: Ashaninka, Aymara, Bribri, Guarani, Nahuatl, Otomi, Quechua, Raramuri, Shipibo-Konibo, and Wixarika.

One odd thing (not sure) is that I had to set

`datasets-cli dummy_data ./datasets/americas_nli/ --auto_generate --n_lines 7500`

`n_lines` very large to successfully generate the dummy files for all the subsets. Happy to get some guidance here.

Otherwise, I hope everything is in order :)

e: missed a step, onto fixing the tests
e2: there you go -- hope it's ok to have added more languages with their ISO codes to `languages.json`, need those tests to pass :laughing: 