src: use a std::vector for preload_modules

A dynamically allocated array was being used, simplify the memory
management by using std::vector.

PR-URL: https://github.com/nodejs/node/pull/12241


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>