process: expose process.features.inspector

Instead of using process.config.variables.v8_enable_inspector
to detect whether inspector is enabled in the build.

PR-URL: https://github.com/nodejs/node/pull/25819
Refs: https://github.com/nodejs/node/issues/25343


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