Convert src/ directory from JavaScript to TypeScript

[Copilot]

This PR converts all JavaScript files in the src/ directory to TypeScript while preserving all existing functionality and keeping JSON configuration files unchanged.

Changes Made

TypeScript Conversion

• Converted 13 JavaScript files to TypeScript with proper type annotations
• Added comprehensive type interfaces including PressureReading, TrendResult, Front, etc.
• Maintained backward compatibility with existing API surface
• Set up complete TypeScript build pipeline with tsconfig.json

Build System Updates

• Added TypeScript compilation scripts to package.json
• Updated main entry point (index.js) to use compiled JavaScript from dist/ directory
• Added prepublishOnly script to ensure TypeScript compilation before package publishing
• Installed required TypeScript dependencies (typescript, @types/node, @types/regression)

JSON Files Preserved

All JSON configuration files remain unchanged as requested:

• src/predictions/fronts.json - Weather front patterns
• src/predictions/prediction_nh.json - Northern hemisphere predictions
• src/predictions/prediction_season.json - Seasonal weather predictions

Test Compatibility

• Updated all test files to import from compiled dist/ directory
• Added special type handling for test data compatibility (e.g., supporting both value and calculated.pressureASL properties)
• All 104 existing tests pass without any functional changes

Type Safety Improvements

The TypeScript conversion adds significant type safety benefits:

// Before (JavaScript)
function getPrediction(pressure, tendency, trend, isSummer = true) {
    let pressureRange = predictionSeasonJSON.find((p) => pressure < p.pressure);
    // No type checking, potential runtime errors
}

// After (TypeScript) 
export function getPrediction(
    pressure: number, 
    tendency: string, 
    trend: string, 
    isSummer: boolean = true
): string | undefined {
    let pressureRange = (predictionSeasonJSON as PressureRange[])
        .find((p) => pressure < p.pressure);
    // Full compile-time type checking
}

Verification

• ✅ All 104 tests pass
• ✅ TypeScript compilation succeeds with no errors
• ✅ Existing API functionality preserved
• ✅ JSON files remain unchanged
• ✅ Package builds and publishes correctly

The conversion maintains 100% backward compatibility while adding the benefits of TypeScript's type system for improved developer experience and code reliability.

Original prompt

Convert the src/ directory from JavaScript to TypeScript. All .json files should be included as-is.

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