Add com-lihaoyi/mainargs?

[bishabosha]

this is probably one of the easier ways to have a proper CLI parsing rather than just positional parameters. unfortunately it does not support nested commands right now (limited to 1 level of subcommands), but it is on all 3 platforms

[lbialy]

so does case-app, which is way more feature-full, I guess we should have a full analysis of pros and cons as before