Implement ZigZag Technical Indicator

[Copilot]

This PR implements the ZigZag technical indicator which helps identify price trends and reversals by filtering out small price movements. The ZigZag implementation:

• Identifies significant price swings based on a percentage threshold
• Tracks high and low swing points to create zigzag lines between important price extremes
• Helps traders identify meaningful price movements by filtering market noise
• Can be customized with different percentage thresholds for various sensitivity levels

The implementation includes:

• ZigZag class (using Big.js for precision)
• FasterZigZag class (using native JavaScript numbers for performance)
• Comprehensive test suite covering various edge cases

Technical Notes

• The implementation tracks "confirmed extremes" that meet the percentage threshold requirement
• Returns null until at least one extreme point is confirmed
• Always returns the last confirmed extreme as the result to ensure continuity

Usage Example

import {ZigZag} from 'trading-signals';

// Create a new ZigZag with 5% threshold
const zigzag = new ZigZag({percentage: 5});

// Add price data
zigzag.update({high: 100, low: 98}, false); // Initial point
zigzag.update({high: 105, low: 103}, false); // Not significant yet
zigzag.update({high: 93, low: 90}, false);   // Significant drop, confirms the previous high

// Get the current ZigZag value
console.log(zigzag.getResult()?.toString()); // Will output the confirmed extreme

Fixes #803.

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