Deprecate chunk_word_count once we move to HybridChunker

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The chunk_word_count parameter (wired all the way through from the ilab CLI) can be deprecated, as we'll no longer be approximating chunks with a word count once we move to the HybridChunker.

We may need some other way to specify chunk size, like a token count, but this issue is just about deprecating chunk_word_count.

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This will likely require corresponding work in the instructlab/instructlab repo to deprecate this within the config and CLI over there.