process: specialize building and storage of process.config

Instead of treating config.gypi as a JavaScript file, specialize
the processing in js2c and make the serialized result a real JSON
string (with 'true' and 'false' converted to boolean values) so
we don't have to use a custom deserializer during bootstrap.

In addition, store the JSON string separately in NativeModuleLoader,
and keep it separate from the map of the builtin source code, so
we don't have to put it onto `NativeModule._source` and delete it
later, though we still preserve it in `process.binding('natives')`,
which we don't use anymore.

This patch also makes the map of builtin source code and the
config.gypi string available through side-effect-free getters
in C++.

PR-URL: https://github.com/nodejs/node/pull/24816


Reviewed-By: Gus Caplan <me@gus.host>