Add initial support for Recursive Chunking (RecursiveChunker)

[bhavnicksm]

This pull request introduces a new RecursiveChunker class and makes several related updates to the codebase. The RecursiveChunker class provides a hierarchical approach to text chunking using customizable rules. Additionally, there are updates to the README.md file, imports, and other existing chunkers.

New Feature:

• RecursiveChunker class: A new class that chunks text hierarchically using customizable rules to create semantically meaningful chunks. This includes methods for splitting text, merging splits, and recursive chunking logic. (src/chonkie/chunker/recursive.py)

Documentation Updates:

• README.md: Added RecursiveChunker to the list of available chunkers and updated the citation format to bibtex. [1] [2]

Import and Export Adjustments:

• __init__.py files: Updated import statements to include RecursiveChunker and related types, ensuring the new class is properly integrated into the module. (src/chonkie/__init__.py, src/chonkie/chunker/__init__.py) [1] [2] [3] [4] [5]

Refinery Enhancements:

• base.py: Added refine_batch method to handle batches of chunks and updated the __call__ method to support both single and batch processing of chunks. (src/chonkie/refinery/base.py) [1] [2]

Other Refinements:

• overlap.py: Improved token handling by introducing _AVG_CHAR_PER_TOKEN and updating methods to use this constant for more accurate token estimates. (src/chonkie/refinery/overlap.py) [1] [2] [3] [4]