Implement Promise then() logic for pending state and Promise.all/race methods

[Copilot]

This PR implements the missing Promise functionality as requested in the issue:

🔧 Promise.then() for Pending Promises

Previously, when calling .then() on a promise in pending state, the callback was ignored and the method returned undefined. Now promises properly queue callbacks and execute them when resolved/rejected:

let resolveCache;
let promise = new Promise(resolve => {
  resolveCache = resolve;
});

// This now works correctly - callback is queued for later execution
let result = promise.then(value => value + ' processed');
resolveCache('Hello');
// result is now a fulfilled promise with "Hello processed"

🔧 Promise.all() Implementation

Added Promise.all() static method that waits for all input promises to resolve, or rejects immediately if any promise rejects:

let p1 = Promise.resolve('First');
let p2 = Promise.resolve('Second');
let p3 = Promise.resolve('Third');

let allResult = Promise.all([p1, p2, p3]);
// Returns fulfilled promise with ["First", "Second", "Third"]

// Rejects immediately if any promise rejects
let withReject = Promise.all([p1, Promise.reject('error'), p3]);
// Returns rejected promise with "error"

🔧 Promise.race() Implementation

Added Promise.race() static method that resolves/rejects as soon as the first input promise settles:

let race1 = Promise.resolve('Winner');
let race2 = Promise.resolve('Too late');

let raceResult = Promise.race([race1, race2]);
// Returns fulfilled promise with "Winner"

🔧 Proper Promise Resolution Logic

Enhanced the promise resolution to correctly handle when promises are returned from .then() handlers, following ECMAScript specifications:

Promise.resolve('start')
  .then(value => Promise.reject(value + ':reject'))
  .then(
    value => Promise.resolve(value + ':fulfilled'), 
    reason => Promise.resolve(reason + ':handled')
  );
// Correctly chains through promise returns

📋 Changes Made

• Added callback storage arrays ([[PromiseFulfillReactions]] and [[PromiseRejectReactions]]) to promise objects
• Modified then() method to queue callbacks for pending promises and return new promises
• Updated resolve() and reject() functions to execute stored callback reactions
• Implemented proper promise resolution logic for returned promises from handlers
• Added Promise.all() and Promise.race() static methods with full ECMAScript compliance
• Added comprehensive test coverage for all new functionality

All existing promise tests continue to pass, ensuring no regression in functionality.

Fixes #26.

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