refactor(types)!: improve type safety and inference of args

[DamianGlowala]

🔗 Linked issue

nuxt/cli#381 inspired me to work on this PR, as I noticed the args can be undefined at runtime even though their types do not reflect this.

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR:

• defines the exact set of available properties based on the arg type
• defines a relationship between required and default properties
• adjust the types of ctx.args.<arg> based on default and required property (currently, ctx.args.<arg> is never undefined)
• inferres the exact possible values for an enum arg when accessing via ctx.args.<arg> (e.g. 1 | 2 | 3 rather than number)
• improves how we handle resolving aliases assigned to parseOptions.alias

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.