deps: split v8_monolith target into separate file

Even if we only use v8_monolith build target, other targets
in v8.gyp with possibly outdated file lists are parsed and
could cause build to fail even with --build-v8-with-gn.

PR-URL: https://github.com/nodejs/node/pull/19201


Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>