feat: show exit code

[ankush]

Implementation:

• Proc handle keeps track of exit code
• Stopped event now passes exit code too, ProcHandle stores this.
• in UI we show exit code if the process actually started and exited, else previous behaviour.

alternative to #118
closes #116

P.S. - I know Rust about as much as that bot, so feel free to close the PR if it doesn't meet the expectation 😄

I love mprocs btw, thanks for this amazing tool!

Edit: Merged, my job is safe from LLMs!

[pvolok]

Thank you!