types: replace declaration merging with mixin types

[flakey5]

For Fastify v5, background: fastify/fastify#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 fastify/fastify#5061 (comment))

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

[climba03003]

I believe if we going to be this approach, we should provides two types file.

1. The plugin types with mixin.
2. The declaration merging types named fastify-global.d.ts.

It allows the users to choice which approach fit there application.

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

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

[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