Add app-desktop module for faster development

[takahirom]

We are developing commonMain. Therefore, I believe it would not be difficult to create a desktop app. I think this would help us iterate more quickly than creating and deploying APKs for Android.

[nyafunta9858]

I wanna challenge this issue! If it is tough for me, I might say give up...

[takahirom]

It's ok! Thank you for your challenge!

[nyafunta9858]

I have opened my pull request! Could you please review this PR?
#452

By the way, I challenged to add a feature module, but it may make my PR to be large because of increasing diffs in a chain reaction.
Therefore I only added the app-desktop module that includes MainApplication which refers to API in the design system module 🙏