feat: add default icon size

[Atinux]

Give the ability to set a default icon size by leveraging the app.config.ts.

Resolves #1

I am not 100% sure about this approach versus using the nuxt.config.ts + public runtime config to give the information back to the component, wdyt @pi0 ?

I do love the DX when having the app.config.ts and see the changes in real-time:

https://user-images.githubusercontent.com/904724/188518312-b37471e2-c369-4b71-a750-a87d91fff056.mp4

Current downside is that I cannot have the autocomplete for the app.config but I guess it's just a typing config that @danielroe can teach me about.

[pi0]

Using app config seems a really nice idea! Some notes:

• Definitely idea better than runtimeConfig (also better alternative to template serialization)
• We need to add meta.contraints.nuxt: '^3.0.0-rc.9 requirement in module spec after this PR to ensure feature is available
• For nuxt built in SEO and pwa, we probably use meta.icon just to mention there is sligh chance of conflicts if we use other top level options like title, etc
• You can manually type schema. Docs: https://v3.nuxtjs.org/guide/features/app-config#manually-typing-app-config