Feature: Add Typescript

[grosuigor]

Add full TypeScript type coverage for Observable/Observer capstone

Summary

This PR migrates the capstone solution from untyped JavaScript to fully typed TypeScript, satisfying the readme acceptance criterion that the entire code sample be covered with types.

• Migrated the implementation from index.js to index.ts with strict typing on Observer, Observable, request handling, and all mock data
• Introduced a modular types/ directory with domain-specific type files and a barrel export via types/index.ts
• Added ObserverLike and ObservableLike contracts so both classes explicitly implement the observer pattern interface
• Added tsconfig.json with strict: true for compile-time safety

Changes

Core implementation (index.ts)

• Observer implements ObserverLike with typed next, error, complete, and unsubscribe methods
• Observable implements ObservableLike with a typed SubscribeFn constructor and subscribe method
• HTTP constants, userMock, requestsMock, and all handler functions are explicitly typed
• Handler return types aligned with actual behavior via HandlerResponse

Type definitions (types/)

File	Types
http.ts	HTTP_METHOD, HTTP_STATUS
user.ts	UserRole, User
request.ts	PostRequest, GetRequest, Request, RequestError, HandlerResponse
handlers.ts	NextHandler, ErrorHandler, CompleteHandler, Handlers
subscription.ts	Unsubscribe, Subscription
observer.ts	ObserverLike
observable.ts	SubscribeFn, ObservableLike, ObservableFactory
index.ts	Re-exports all public types

Notable typing decisions

• Discriminated request union — PostRequest requires a typed User body; GetRequest requires params, preventing invalid request shapes at compile time
• RequestError — structured error type replacing any
• UserRole — narrowed to 'user' | 'admin' instead of generic string[]
• Optional _unsubscribe — satisfies strictPropertyInitialization without weakening type safety

Tooling

• Added tsconfig.json with strict mode enabled
• Removed legacy index.js in favor of the typed implementation

Test plan

• Run npx tsc --noEmit — compiles with zero errors under strict mode
• Verify no TypeScript/linter diagnostics in index.ts and types/
• Confirm Observer and Observable satisfy their *Like interfaces
• Confirm requestsMock type-checks as Request[] (POST with User body, GET with params)
• Confirm handlers return HandlerResponse with valid HTTP_STATUS values
• Run node index.ts (or equivalent) and verify complete / unsubscribed log output

Acceptance criteria

• Observer and Observable are fully typed
• Request objects are fully typed
• User objects are fully typed
• Observer handlers are fully typed
• Entire code sample is covered with types (no any)