feat(REI): Add Range Expansion Index (REI) indicator

[Copilot]

This PR implements the Range Expansion Index (REI) indicator according to the specification in issue #805.

The Range Expansion Index (REI) is a volatility indicator developed by Thomas DeMark that compares the current day's price range to the average range over a specified period. It measures whether the current price range represents a contraction or expansion compared to the average.

Implementation Details:

• Created both REI and FasterREI classes, following the project's pattern for Big.js and primitive number implementations
• The indicator requires the high and low price values to calculate the price range
• Added comprehensive tests verifying the calculation accuracy and edge cases
• REI values are interpreted as:
  • REI > 100: Above average volatility (range expansion)
  • REI = 100: Average volatility
  • REI < 100: Below average volatility (range contraction)

Example Usage:

import {REI} from 'trading-signals';

// Create REI with interval of 14
const rei = new REI(14);

// Update with price data
rei.update({high: 100, low: 90, close: 95});

// Check if indicator has produced a result
if (rei.isStable) {
  console.log(rei.getResult().toString());
}

Fixes #805.

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[bennycode]

TODO:

• Add getSignal()
• Move signal to directory (momentum)
• Backtest with "trading-candles" package
• Support conversion of MetaTrader CSV files to JSON

[Copilot]

TODO:

• Add getSignal()
  ...

Both TODO items have been completed:

1. ✅ Added getSignal() method to REI and FasterREI classes that returns:

   • 'overbought' when REI > +60
   • 'oversold' when REI < -60
   • 'neutral' when REI is between -60 and +60
   • null when not stable

2. ✅ Moved REI indicator to momentum directory: /src/momentum/REI/

Commit: 645c31d