[fsmt tokenizer] support lowercase tokenizer
This PR:

* [x] implements support for `do_lower_case` in the fsmt tokenizer - requested [here](https://github.com/huggingface/transformers/pull/8374#issuecomment-723419221)
* [x] adds test to validate the new feature
* [x] adds case detector in the converter script

@LysandreJik 