src: guard against calling `Init()` multiple times

This function should only be called once. Calling it multiple times
would currently break Node.js (e.g. re-registering builtin modules
would break the linked list for them).

PR-URL: https://github.com/nodejs/node/pull/26458


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>