src: allow inspector without v8 platform

Embedders sometimes want to take control of V8 platform initialization
themselves, so we wouldn't want to experience an error if
NODE_USE_V8_PLATFORM was false necessarily. Instead, we can also gate
it with HAVE_INSPECTOR to allow embedders to define that themselves
and use Inspector without NODE_USE_V8_PLATFORM.

PR-URL: https://github.com/nodejs/node/pull/30049


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>