src: add READONLY_STRING_PROPERTY and simplify config

Bit of tidying up where we set different config values.

PR-URL: https://github.com/nodejs/node/pull/22222


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>