feat: Add pingpong and fract functions

[oglego]

Summary

This PR introduces two new mathematical functions to the extension: pingpong and fract.

Mathematical Implementation

• fract(x): Returns the fractional (decimal) part of a number.

• pingpong(x, min, max): Returns a value that "bounces" back and forth between the minimum and maximum.

Key Changes

• New Functions: Added pingpong and fract.
• Vectorized Execution: Leveraged DuckDB's native executors for high-performance processing.
• Documentation: Updated README.md with usage descriptions and examples.
• Testing: Added comprehensive unit tests covering:
  • Positive and negative inputs for fract.
  • Range boundaries and large inputs for pingpong.

Usage Examples

-- Returns 0.25
SELECT fract(42.25);

-- Returns 8.0 (Value bounces back from 10)
SELECT pingpong(12, 0, 10);