bugfix: add missing limits.parts type

[Curti-s]

When registering the @fastify/multipart plugin, esp in TS projects, one cannot be able to provide a limit for parts config.
I believe that, that's not deliberate and that it's also a bug, which this PR addresses.

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

[mcollina]

Can you please add a test for this type? We use tsd to test types

[gurgunday]

I took a look at the test-d file, and I still don't get how we test a config property – just by passing an incorrect type?

[Eomm]

YOu can add the test here:

fastify-multipart/test/index.test-d.ts

Line 24 in 80a01f4

limits: {