Use process.exitCode instead of explicitly using process.exit

In `aio app run`, we have a SIGINT handler, and we explicitly call `process.exit` after trying to clean up our resource usage. We should just set the appropriate exit code and let the program naturally exit.

However, something is not being cleaned up properly, thus the program does not exit. Will need to investigate the issue further.

See discussion here: https://github.com/adobe/aio-cli-plugin-app/pull/343#discussion_r560450510

Start with the SIGINT handling used by `aio app run` and review the discussion in PR #343. Investigate what resource remains active after setting `process.exitCode`; done means the command exits naturally after cleanup with the appropriate exit code.

Written by the indexing model from the issue text.