Add `get_added_tokens_decoder`, `token_to_id`, and `id_to_token` functions

[xenova]

Add get_added_tokens_decoder function, as well as token_to_id and id_to_token

Example python usage:

tokenizer.token_to_id("hello") # 31373
tokenizer.id_to_token(31373) # "hello"
tokenizer.get_added_tokens_decoder() # {50256: AddedToken("<|endoftext|>", rstrip=False, lstrip=False, single_word=False, normalized=False, special=True, ...}