src: simplify native immediate by using v8::Global

Unlike `node::Persistent`, `v8::Global` has move semantics and
can be used directly in STL containers.

PR-URL: https://github.com/nodejs/node/pull/26254


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>