fix: process sometimes not exiting on node 15+

[alexbchr]

Fixes #243 by calling process.exit(0) to ensure the process correctly exits after a the successful execution of the cli function.

As mentioned by @ricokahler, Next.js does the same in their codebase.

[marconucara]

Can you merge this please?

[lbittner-pdftron]

+1 Can we please get this merged

[smoglica]

Same issue, +1 for merging

[felixgren]

Let us start 2023 strong by getting this merged

@ricokahler