Add async/await and Promises examples to ECMAScript 2021

Added comprehensive async/await and Promises documentation and examples:
- Created asyncawait.js with 7 detailed examples covering basic Promises, async/await syntax, Promise.all(), sequential operations, error handling, Promise.race(), and Promise chaining
- Updated README.md with new async/await section explaining Promises, their states, benefits, and usage patterns in ServiceNow
- All examples use ServiceNow-specific GlideRecord operations to demonstrate real-world scenarios

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude