process_close(): uv_unref() detached processes (#7539)

Doc for UV_PROCESS_DETACHED in uv.h mentions:
> child process will still keep the parent's event loop alive unless
> the parent process calls uv_unref() on the child's process handle.

ref #3944

src/nvim/event/process.c

@@ -324,6 +324,13 @@ static void process_close(Process *proc)
   }
   assert(!proc->closed);
   proc->closed = true;
+
+  if (proc->detach) {
+    if (proc->type == kProcessTypeUv) {
+      uv_unref((uv_handle_t *)&(((LibuvProcess *)proc)->uv));
+    }
+  }
+
   switch (proc->type) {
     case kProcessTypeUv:
       libuv_process_close((LibuvProcess *)proc);