src: make BaseObject::is_snapshotable virtual

Use a virtual function in order to save space (8 bytes per instance
on 64-bit platforms) and in order to be consistent with the other
methods on BaseObject.

PR-URL: https://github.com/nodejs/node/pull/37539


Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>