src: properly configure default heap limits

Unless configured, V8 defaults to limiting the max heaps size to 700 MB
or 1400MB on 32 and 64-bit platforms respectively. This default is
based on the browser use-cases and doesn't make a lot of sense
generally. This change properly configures the heap size based on
actual available memory.

PR-URL: https://github.com/nodejs/node/pull/25576


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Yang Guo <yangguo@chromium.org>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>