feat: add comprehensive Modern JavaScript ES6+ syntax concept page

- Complete rewrite covering arrow functions, destructuring, spread/rest,
  template literals, optional chaining, nullish coalescing, Map/Set/Symbol
- Add 57 tests covering all major ES6+ features
- Include fact-checked code examples with correct edge case behavior
- Add curated resources: 6 MDN refs, 6 articles, 4 videos

Author: leonardomso