Add Semantic Chunking Support (LangChain SemanticChunker)

Add Semantic Chunking as a chunking option in Lamatic. It's available in LangChain's experimental module via SemanticChunker and splits text based on meaning rather than fixed character/token counts — making it much better for RAG use cases where chunk quality directly impacts retrieval accuracy.

Worth testing as an optional chunking strategy alongside the existing splitters.