Add saving and loading corpus/stopwords to `Tokenizer` and add integration to HF Hub via `bm25s.hf.TokenizerHF` (save/load)

[xhluca]

• Add Tokenizer.save_vocab and Tokenizer.load_vocab methods to save/load vocabulary to a json file called vocab.tokenizer.json by default
• Add Tokenizer.save_stopwords and Tokenizer.load_stopwords methods to save/load stopwords to a json file called stopwords.tokenizer.json by default
• Add TokenizerHF class to allow saving/loading from huggingface hub
  • New function: load_vocab_from_hub, save_vocab_to_hub, load_stopwords_from_hub, save_stopwords_to_hub