Support for CLI

[shinGangan]

Describe the feature

Context

I am currently using unjs/citty to make CLI tools. While creating CLI tools, I have been saved by many unjs stacks such as

• 🌆 unjs/citty
• 🐨 unjs/consola
• 💅 unjs/changelogen
• 📦 unjs/unbuild
• 🍟 unjs/jiti

There are others that are needed(magicast, unctx, c12, pathe, etc), but we believe the above are the minimum. So, I would like to provide unkit for those who want to make CLI with unjs in the future.

Design

• add src/cli.ts
• add exports.cli options in package.json

Additional information

• [X] Would you be willing to help implement this feature?

### Tasks

[shinGangan]

Hi @pi0 ( and @NozomuIkuta ).

I would appreciate a review when you have time. I'll create a Pull Request if you're okay with the policy.