src: create per-isolate strings after platform setup

Allocation of strings may cause a garbage collection that uses
the platform to post tasks.

PR-URL: https://github.com/nodejs/node/pull/20175
Fixes: https://github.com/nodejs/node/issues/20171


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yang Guo <yangguo@chromium.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>