types: replace declaration merging with mixin types

[flakey5]

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For Fastify v5, background: https://github.com/fastify/fastify/issues/5061

Opened this as it is now to get early feedback & insight. Used a class rather than an interface or type since the type needs to exist at runtime if we go with the currently proposed changes to the fastify-plugin api (see https://github.com/fastify/fastify/issues/5061#issuecomment-1874592015)

TODO:

• [ ] Update/check tests.

cc @mcollina

Checklist

• [ ] run npm run test and npm run benchmark
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [ ] commit message and code follows the Developer's Certification of Origin and the Code of conduct

[voxpelli]

The second option can be consumed using the tsconfig.json by the following way.

{
  "compilerOptions": {
    "types": ["@fastify/static/fastify-global"]
  }
}

This only works for top level projects. I would much prefer it to be handled through an import {} from '@fastify/static/fastify-global', especially as specifying compilerOptions.types disables automatic loading of all @types/ types