Microsoft BitNet Hugging Face configs use BitNetForCausalLM while the
converter only registered BitnetForCausalLM, causing conversion to fail
with "Model BitNetForCausalLM is not supported".
Register both spellings in TEXT_MODEL_MAP and the Bitnet model class.
Fixesggml-org/llama.cpp#25629
* move conversion code to a dedicated conversion directory and split the files akin to the src/models architecture
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Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
