feat: add comprehensive ES Modules concept page with tests

- Complete rewrite of ES Modules documentation covering:
  - Why ES Modules matter (static analysis, tree-shaking)
  - ESM vs CommonJS comparison with detailed table
  - Live bindings explanation with ASCII diagrams
  - Export syntax (named, default, re-exports, barrel files)
  - Import syntax (named, default, namespace, side-effect)
  - Module characteristics (strict mode, scope, singleton)
  - Dynamic imports with real-world use cases
  - Top-level await patterns
  - Browser vs Node.js differences
  - Import maps for bare specifiers
  - Common mistakes section

- Added 57 comprehensive tests covering:
  - Live bindings behavior vs CommonJS value copies
  - Read-only import bindings
  - Circular dependencies and TDZ (ReferenceError)
  - Module singleton behavior
  - Dynamic import patterns
  - Export/import syntax variations
  - Module characteristics (strict mode, this, scope)
  - Common mistakes (named vs default, extensions)

Author: leonardomso