feat: code chunking backend for docling

[bridgetmcg]

This PR introduces support for parsing and chunking source code files in Docling, enabling structured processing of programming languages with intelligent function-level chunking capabilities. It requires docling-core PR 398 be merged for testing to work.

Features

• Multi-language support: Python (.py), JavaScript (.js), and Java (.java) files
• Unified code representation using Docling's document model
• Integration with existing DocumentConverter
• Pipeline configuration using SimplePipeline for code processing

Testing

• Basic Java function chunking functionality

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

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