feat: track token provenance

[stephantul]

Fixes #209 (again 😓 )

This PR adds a Token class to keep track of whether a token was originally a subword or added token. If it was a subword token, we should not pretokenize it. Previously, we relied on whether a token was in the original vocabulary, but this turned out to not work for unigram tokenizers with a metaspace if the token was also a subword token.

For "ELLE", for example:

1. Check if "ELLE" is in vocab, we pretokenize it to "_ELLE". This is fine
2. Then, when adding it, we need to check whether it is a subword. But "ELLE" is also a subword.
3. So we don't pretokenize it before adding it, causing us to add two tokens with the same surface form.

[codecov]

Codecov Report

Attention: Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
model2vec/distill/tokenizer.py	62.50%	3 Missing ⚠️

Files with missing lines	Coverage Δ
model2vec/distill/distillation.py	94.24% <100.00%> (ø)
model2vec/distill/inference.py	96.05% <100.00%> (+0.05%)	⬆️
model2vec/distill/utils.py	100.00% <100.00%> (ø)
model2vec/distill/tokenizer.py	52.00% <62.50%> (+0.64%)	⬆️

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[Pringled]

LGTM