Add Making your own NestJS on top of express under 180 lines

[sjsakib]

This PR adds a guide on building a minimal NestJS-like framework from scratch to explain how NestJS works internally.

The article walks through:

• How decorators attach metadata to classes, methods, and parameters
• How routing can be implemented by reading method metadata at runtime
• How constructor-based dependency injection works using design:paramtypes
• How parameter decorators (e.g. @Param) enable data injection into route handlers

The implementation intentionally stays small and readable (~180 LOC) and avoids abstractions not essential to understanding the core ideas. All examples are runnable, and the final result wires everything together on top of Express.

Why this belongs in build-your-own-x

NestJS is widely used but often feels “magical” to newcomers. This guide:

• Demystifies NestJS internals using plain TypeScript
• Focuses on mechanisms, not APIs
• Is suitable for readers already familiar with TypeScript and decorators