chore: remove tsd

[mrazauskas]

Checklist

• run npm run test && npm run benchmark --if-present
• 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

---

The context on why tsd should be replaced: fastify/fastify#6033.

I think in this case it can be simply removed without introducing another type testing library.

• There is only one expectType(), but the tsd command is not running.
• This means that expectType() acts the same as expectAssignable(), because it is only checked by tsc and both assertions have the same type definitions (reference, reference).
• Assignability is already checked by tsc in the next line under the assertion.
• The build:test script makes sure all test files are type-checked; this can be verified by adding a failing line and running npm run build:test.

Or do I miss something?

[mcollina]

lgtm

[mcollina]

can you fix the conflicts?

[mrazauskas]

@mcollina The branch is rebased. Please have a look.

[mcollina]

lgtm