refactor: modernize MulterError to ES6 class

[Phillip9587]

This PR modernizes the MulterError implementation by converting it from a function constructor with the legacy util.inherits() pattern to an ES6 class.

[Phillip9587]

Is not needed anymore as the stack trace is automatically captured by the constructors super call.

[Copilot]

Pull Request Overview

This PR modernizes the MulterError implementation by converting it from a function constructor pattern to an ES6 class syntax, eliminating the need for util.inherits() and adopting contemporary JavaScript patterns.

• Converts function constructor to ES6 class extending Error
• Removes util.inherits() dependency and util import
• Updates variable declaration from var to const for errorMessages

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[Copilot]

The Error.captureStackTrace() call was removed but not replaced. In ES6 classes extending Error, you should call Error.captureStackTrace(this, MulterError) after super() to maintain proper stack trace behavior.

Suggested change

	super(errorMessages[code])
	super(errorMessages[code])
	Error.captureStackTrace(this, MulterError)

[Phillip9587]

I ran a short test using the following script:

// ORIGINAL IMPLEMENTATION
function MulterErrorOriginal(message) {
  Error.captureStackTrace(this, this.constructor);
  this.name = this.constructor.name;
  this.message = message;
}
require('util').inherits(MulterErrorOriginal, Error);

// NEW ES6 CLASS IMPLEMENTATION (with Error.captureStackTrace) 
class MulterErrorNew extends Error {
  constructor(message) {
    super(message);
    this.name = this.constructor.name;
    Error.captureStackTrace(this, MulterErrorNew);
  }
}

// NEW ES6 CLASS IMPLEMENTATION (without Error.captureStackTrace)
class MulterErrorNewNoCapture extends Error {
  constructor(message) {
    super(message);
    this.name = this.constructor.name;
  }
}

function testErrorImplementation(ErrorClass, name) {
  console.log(`\n${'='.repeat(50)}`);
  console.log(`${name}`);
  console.log(`${'='.repeat(50)}`);
  
  // Basic properties test
  const err = new ErrorClass('LIMIT_FILE_SIZE', 'avatar');
  console.log('Properties:');
  console.log('  name:', err.name);
  console.log('  instanceof Error:', err instanceof Error);
  console.log('  instanceof ErrorClass:', err instanceof ErrorClass);

  try {
    throw new ErrorClass('LIMIT_FILE_COUNT');
  } catch (error) {
    console.log(error.stack);
  }
}

console.log('Node.js version:', process.version);
console.log('Comparing MulterError implementations...');

// Test all three implementations
testErrorImplementation(MulterErrorOriginal, 'ORIGINAL (function + util.inherits)');
testErrorImplementation(MulterErrorNew, 'NEW ES6 CLASS (with Error.captureStackTrace)');
testErrorImplementation(MulterErrorNewNoCapture, 'NEW ES6 CLASS (without Error.captureStackTrace)');

As expected all outputs are the same so this comment is wrong:

Node.js version: v10.16.3
Comparing MulterError implementations...

==================================================
ORIGINAL (function + util.inherits)
==================================================
Properties:
  name: MulterErrorOriginal
  instanceof Error: true
  instanceof ErrorClass: true

MulterErrorOriginal: LIMIT_FILE_COUNT
    at testErrorImplementation (/home/pbarta/repos/expressjs/multer/compare-implementations.js:47:11)
    at Object.<anonymous> (/home/pbarta/repos/expressjs/multer/compare-implementations.js:57:1)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:622:3)

==================================================
NEW ES6 CLASS (with Error.captureStackTrace)
==================================================
Properties:
  name: MulterErrorNew
  instanceof Error: true
  instanceof ErrorClass: true

MulterErrorNew: LIMIT_FILE_COUNT
    at testErrorImplementation (/home/pbarta/repos/expressjs/multer/compare-implementations.js:47:11)
    at Object.<anonymous> (/home/pbarta/repos/expressjs/multer/compare-implementations.js:58:1)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:622:3)

==================================================
NEW ES6 CLASS (without Error.captureStackTrace)
==================================================
Properties:
  name: MulterErrorNewNoCapture
  instanceof Error: true
  instanceof ErrorClass: true

MulterErrorNewNoCapture: LIMIT_FILE_COUNT
    at testErrorImplementation (/home/pbarta/repos/expressjs/multer/compare-implementations.js:47:11)
    at Object.<anonymous> (/home/pbarta/repos/expressjs/multer/compare-implementations.js:59:1)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:622:3)

[coveralls]

coverage: 98.467% (-0.02%) from 98.485%
when pulling 57b6e99 on Phillip9587:error-class
into b6e4b1f on expressjs:main.