Don't use process.exit in CLI to prevent truncated stdout

[jakebailey]

I'm working on the type completeness dashboard and hit nodejs/node#6379 (and associated issues) when capturing pyright's output.

Avoid using process.exit to stop the process, instead exiting gracefully with the exit code set. process.exit synchronously exits the process, but doesn't flush console's internal buffers or stdout. This can lead to a truncated output, especially when the output is large (which is the case with --verifytypes and many libraries). There isn't a good way to force a flush (besides calling undocumented functions to put stdout into blocking mode), so avoiding process.exit is a better method and fits with the NodeJS docs that say process.exit is not recommended.

After #1822 fixed the file watcher hang, this should be safe to do, but does rely on all promises resolving before the process can exit.

[erictraut]

Ah, good find. I didn't realize that process.exit exited without flushing. That explains some problems I've experienced in other node-based command-line tools I've written in the past :).

The changes LGTM.