inspector: split main thread interface from transport

Workers debugging will require interfacing between the "main" inspector
and per-worker isolate inspectors. This is consistent with what WS
interface does. This change is a refactoring change and does not change
the functionality.

PR-URL: https://github.com/nodejs/node/pull/21182
Fixes: https://github.com/nodejs/node/issues/21725


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>