Enforce callback signatures with C++ Concepts

[legendecas]

#1738 documented the required signature of the templated finalizer callback parameters as inline comment hints.

These are human-only annotations. The compiler does not enforce them, so a mismatched callable fails deep inside the template instantiation with an unhelpful error.

Once C++20 is the minimum supported standard, we should replace these hints with C++ Concepts that constrain the Finalizer/FinalizerType template parameters. This gives compiler-enforced contracts and clear diagnostics at the call site.

Finalizer candidates:

• BasicEnv::PostFinalizer
• Object::AddFinalizer
• External::New
• Buffer::New / Buffer::NewOrCopy
• ThreadSafeFunction::New
• TypedThreadSafeFunction::New

Callback candidates:

• Function::New — the JS callback (Callable)
• PropertyDescriptor::Accessor — Getter / Setter
• PropertyDescriptor::Function — Callable
• ThreadSafeFunction::BlockingCall / NonBlockingCall — Callback
• TypedThreadSafeFunction::BlockingCall / NonBlockingCall — Callback
• TypedThreadSafeFunction — the CallJs callback