types!: add createPlugin helper for registration-scoped typing

[mcollina]

Summary

This PR introduces a typed createPlugin() helper in fastify-plugin to
support the registration-scoped typing flow used by the declaration-merging
removal migration.

It adds:

• createPlugin() runtime export;
• TypeScript typings and examples for createPlugin();
• tests covering runtime dependency metadata handling for createPlugin().

Context / previous attempts

This continues the work discussed in prior attempts around typed plugin effect
transport:

• Support decorators #247

Dependency on Fastify core migration

This PR is part of the migration stack and depends on Fastify core changes in:

• types!: remove declaration-merging reliance with registration-scoped decorator typing fastify#6581 (feat/typed-decorators)

Please review/merge in dependency order.

Validation

• npm run lint ✅
• npm run test:unit ✅
• npm run test:typescript ✅

Issue

Closes #247.