feat(module): improve app config types autocomplete

[davestewart]

🔗 Linked issue

Resolves #1869

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ 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 enables full component config auto-completion in app.config.ts:

The existing Utility class used object and so auto-completion would fail below the top level.

This new implementation of DeepPartial:

• uses recursive typing to supply full auto-complete of nested properties
• allows custom properties to be added, for example in variant
• allows new properties to be optionally constrained, in this case to string

It seems this is the only usage of DeepPartial in the project, so I assume it won't mess up anything else.

📝 Checklist

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

[davestewart]

@benjamincanac, one thing to note...

In the original DeepPartial (which does work, sorry; I should have looked more closely) known properties are suggested like this:

In the version I propose, the typing is tighter:

I suspect your TS-fu is greater than mine... but worth mentioning.

[benjamincanac]

It does look better with your suggestion! Thanks 😊