chore: upgrade to typescript v6.0.2 and others updated

[Tony133]

Proposals:

• Upgrades TypeScript to v6, modernizes the compiler configuration, and refactors the flash plugin implementation to be compatible with stricter type checking and node16 module resolution.

  • Changes made

    • Update tsconfig.json:
      • Updated moduleResolution from node to node16
      • Updated target from es2017 to es2022
      • Updated module from commonjs to node16
      • Removed experimentalDecorators and emitDecoratorMetadata ( no longer needed )

  • Update tsconfig.build.json

    • Refactored to extend tsconfig.json and add compilerOptions for build output (outDir: dist, rootDir: src)
    • Replaced include: ["./src/**/*.ts"] with include: ["src"]

• Update src/index.ts:

  • Added explicit FastifyRequest and FastifyReply type annotations to plugin methods, replacing implicit this typing
  • Added FlashData type alias for { [k: string]: string[] }
  • Exported ReplyReturn type (was previously internal)
  • Added module augmentation for @fastify/secure-session to declare SessionData with flash field
  • Simplified session handling: replaced manual null checks with nullish coalescing (|| {})
  • Refactored reply() method to use destructuring for cleaner flash message removal
  • Fixed return type of reply() to use ?? 0 instead of || [] for empty flash count

see reference PR: fastify/fastify#6605