Wrap all Node-API functions in neon-runtime to check status

Currently every usage of a Node-API function checks the status. This is almost always required for safety, but it's easy to fix.

Since all Node-API FFI bindings are defined in a macro and all functions return napi_status, we could wrap them to return Result<(), Status> instead and use the linter to ensure the Result is checked.

mod node_api {
    extern "C" {
        fn get_undefined(env: Env, result: *mut Value) -> Status;
    }    
}

pub unsafe fn get_undefined(env: Env, result: *mut Value) -> Result<(), Status> {
    match node_api::get_undefined(env, result) {
        Status::Ok => Ok(()),
        status => Err(status),
    }
}