Migrate CLI from Caporal.js

Caporal is [unmaintained](https://github.com/mattallty/Caporal.js/issues/243) at this point, and the outdated dependencies underneath it are piling up. Especially now that `@gltf-transform/cli` offers an experimental plugin API, it'd be nice for the interface to custom CLI commands to be stable. Some alternatives:

- https://github.com/oclif/oclif
- https://www.npmjs.com/package/yargs
- and https://github.com/bhouston/yargs-file-commands
- https://github.com/tj/commander.js/

~~I'm leaning toward oclif.~~

The issue does not name any files, tests, or CLI entry points, and it leaves the replacement undecided among oclif, yargs, and Commander. Start by locating the current Caporal integration and the custom-command interface, then compare the alternatives before defining the migration scope and completion criteria.

Written by the indexing model from the issue text.