[FEATURE] Allow users to customize separator patterns for `SplitRecursively`

[liemkg1234]

What is the use case?
I want to add my custom Splitter likely cocoindex.functions.SplitRecursively()
I want to custom with @op.executor_class

Describe the solution you'd like
My code when use with llama-index

   # Merge small chunks
    chunks = merge_small_chunks(chunks, min_size=256)

    # Sentence Splitter
    splitter2 = CustomSentenceSplitter(
        chunk_size=chunk_size,
        chunk_overlap=0,
        separator="\n",
        paragraph_separator="\n\n\n",
        secondary_chunking_regex="[^,.;。？！]+[,.;。？！]?",
    )
    chunks = splitter2(chunks)

    # Context Retrieval
    chunks = process_nodes(chunks, max_workers=10)
    return chunks

Additional context

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[badmonster0]

For this case, sounds like we can reuse SplitRecursively while allow customizations with additional options (for separators)

[badmonster0]

This is not supported. Please see the new custom_languages field for SplitRecursively: https://cocoindex.io/docs/ops/functions#splitrecursively