Add support for FixedLength pretokenizer

[xenova]

Used by https://huggingface.co/onnx-community/Supertonic-TTS-ONNX

rust version: https://github.com/huggingface/tokenizers/blob/main/tokenizers/src/pre_tokenizers/fixed_length.rs
rust PR: huggingface/tokenizers#1713

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Transformers.js version implemented already: huggingface/transformers.js#1459
next steps: implement this here too.