fix: migrate test suite from tap to node:test

[nimesh0505]

This PR replaces tap with node:test and introduces c8 for code coverage.

Checklist

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

[simoneb]

Do we need c8 or can we use the built-in coverage? https://nodejs.org/api/test.html#collecting-code-coverage

[nimesh0505]

Do we need c8 or can we use the built-in coverage? https://nodejs.org/api/test.html#collecting-code-coverage

@simoneb Thanks for suggesting the use of Node.js's built-in coverage. After consideration, I think we should stick with c8 for now. Here's why:

1. The --experimental-test-coverage flag is still experimental, which could lead to instability or changes in future Node.js versions.
2. As a widely-used package, @fastify/pre-commit should prioritize stability for its users.
3. c8 is a mature, stable solution that we know works well across different Node.js versions.

However, I agree it's a good idea to eventually move to the built-in coverage. I suggest we:

1. Keep using c8 for now.
2. May be create an issue to track the eventual migration to Node.js's built-in coverage.
3. Revisit this once the coverage feature in Node.js becomes stable (likely when they remove the experimental prefix).

This way, we maintain stability for our users while staying prepared to adopt the built-in solution when it's ready. What do you think?

[simoneb]

Sounds good, I brought it up in case other maintainers have an opinion on this

[mcollina]

Use c8 for now.

[climba03003]

Can you change to use the built-in t.plan and t.assert?