[v4] Tokenizers.js migration (#1501)
* Remove legacy tokenizer tests
* Update utils.test.js
* Update unit tests
* Refactor tokenizers.js
* Update streamers.js
* Update imports
* Initial tokenization migration
* Remove unnecessary index.js files
* Do not export PretrainedMixin
* Refactor models.js/tokenizers.js
* Update imports
* Update more imports
* Update folder structure and imports
* Delete tokenization_code_xlm_roberta.js
* Fix import path
* Fix typos
* Update import path
