Refactor: Implemented Modular Vectorized Backtesting Engine (OOP)

[Rakeshks7]

I noticed the repository currently relies on standalone scripts with duplicated backtesting logic. To improve maintainability and scalability, I have introduced a unified VectorizedBacktester base class.

Key Changes:

• OOP Architecture: Created a base parent class VectorizedBacktester in core/ to handle PnL, transaction costs, and plotting.
• Performance: Switched from iterative loops to vectorized Pandas operations for faster execution.
• Standardization: Added a get_performance_metrics() method to standardize Sharpe/Drawdown reporting across all future strategies.
• Proof of Concept: Ported the MACD strategy to this new framework as a demonstration.