refactor(types): move index.d.ts and rename test-types

[Tony133]

Proposal:

• Moved index.d.ts into types/
• Renamed test-types/ to types/
• Updated package.json accordingly
• Kept the existing type tests under the new structure

Note:

• This reorganizes the type definitions and type tests into a single types/ directory, keeping the project structure consistent with the other repositories.
• Ref PR: refactor(types): move index.d.ts and rename test-types fastify-postgres#233