feat: Design-time command tree checker

[carnesen]

As a TypeScript-first library, we mostly rely on compiler type checking to ensure correctness of the user's CLI. It would be nice for our non-TypeScript users if we could provide some kind of "checker" function that would walk a command tree and inspect all the nodes for correctness. This would be used as a design-time inspection called during unit tests or whatever.

Some checks:

• branch.subcommands.length >=1
• Subcommand name conflicts
• No top-level option "version" (conflicts with built-in)
• No option "help" (conflicts with built-in)

[carnesen]

Howdy yall! This project was originally a copy-paste fork of @carnesen/cli, which I've continued to work on over the last six months. I just put out the 0.5.0 release including thorough online documentation and examples. @carnesen/cli is also fully isomorphic, so it works in a browser too! Check it out and star the project's repo on GitHub if you like what you see. Since that's the more actively maintained fork of this project, I'll close my feature requests here. As always, please just let me know by GitHub or email if there's anything I can do to help with any of my open-source contributions.