src: avoid duplicate Before/AtExitCallback structs

Currently, BeforeExitCallback and AtExitCallback are identical apart for
the name of the struct. This commit introduces an ExitCallback struct
with can be used in both cases to avoid the duplication.

PR-URL: https://github.com/nodejs/node/pull/19226


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>