Refactor CLI parsing for synthesis entrypoint

[shadaj]

Related to #3

This pull request simplifies the CLI parsing logic for the synthesis entrypoint in tests/synthesize_crdt.py and enhances the user experience by utilizing the argparse library.

• Replaces manual sys.argv parsing with structured command-line options using argparse, making the script more user-friendly and maintainable.
• Introduces a main function to encapsulate the script logic, improving readability and structure.
• Adds argument parsing with options for synthesis mode, benchmark selection, and additional flags (--fixed, --first, --repeat), allowing users to easily specify their synthesis requirements.
• Implements help messages for each command and argument, automatically generated by argparse, providing users with clear guidance on how to use the script.
• Updates the README.md to reflect the new argument structure, ensuring documentation is consistent with the code changes.

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