fix(types): export function definition types

[simonepizzamiglio]

Fixes #12.

While working on one of my private projects, I encountered the same issue described in #12.
I’m using deepmerge with TypeScript, and when building my project, I experienced the heap out of memory error mentioned in that issue.

To resolve this, I tried exporting DeepMergeFn and DeepMergeAllFn directly from the @fastify/deepmerge folder in node_modules, which successfully fixed the problem.

Alternatively, if the issue persists, you can leverage these types as follows:

const deepmergeAll: DeepMergeAllFn = deepmerge({ all: true });

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]

Thanks for opening a PR! Can you please add a unit test? We use tsd for types.

[simonepizzamiglio]

Thanks for opening a PR! Can you please add a unit test? We use tsd for types.

@mcollina Tests added 🙂

[Uzlopak]

I guess exporting the types wont harm ;)