fix(ext/napi): export uv_ref and uv_unref symbols

[bartlomieju]

Summary

• Adds no-op uv_ref and uv_unref stubs to ext/napi/uv.rs
• Exports them in the symbol lists for all platforms

Native addons (e.g. zeromq) that call uv_ref/uv_unref crash with a null
pointer dereference because these symbols are resolved via dlsym at runtime
and Deno didn't export them.

Reproduction

import zmq from "zeromq";
const sock = new zmq.Pair();
await sock.bind('tcp://127.0.0.1:3000');

Crashes with EXC_BAD_ACCESS (code=1, address=0x0) — the call chain is:

napi_register_module_v1
  → zmq::Module::Module
    → zmq::Trash<...>::Trash
      → uv_unref(handle)  // resolved to NULL → crash

Status

This fixes the immediate uv_unref crash, but zeromq also needs additional uv
functions to fully work: uv_poll_init_socket, uv_poll_start, uv_poll_stop,
uv_timer_init, uv_timer_start, uv_timer_stop, uv_idle_init,
uv_idle_start, uv_check_init, uv_check_start, uv_os_getpid,
uv_queue_work.

There is a more complete uv compat layer in libs/core/uv_compat.rs (with
timers, idle/check/prepare handles, etc.) gated behind the uv_compat_export
feature flag. Integrating that with the NAPI layer's existing uv polyfill in
ext/napi/uv.rs would be the path to full zeromq support.

Ref #18345

Test plan

• cargo check -p deno_napi passes
• Confirmed zeromq gets past the uv_unref crash (now fails later on
  missing uv_poll_init_socket)

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