src: add abstract `IsolatePlatformDelegate`

Adds a new abstract class for module authors and embedders to register
arbitrary isolates with `node::MultiIsolatePlatform`.

PR-URL: https://github.com/nodejs/node/pull/30324


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>