Improvements for validation settings template

[avocadomayo]

Background

Related #103
Follows #134

Solution

This PR does two things:

• Fix a bug with the example validation settings app
  • We previously checked api.data.validation?.metafields to see if a metafield definition exists. However, this field is a bit misleading such that it evaluates to [] when the definition does not exist OR definition exists and no value is defined. As a result, we end up creating the metafield definition on each render
  • Add a metafieldDefinitions query to explicitly check for existing metafield definitions, and only create a new definition if there is truly no existing definition
• Remove layout component BlockStack, no longer necessary since we provide the layout directly in FunctionSettings

Checklist

• I have 🎩'd these changes
• I have squashed my commits into chunks of work with meaningful commit messages

[aledalgrande]

Makes sense ✨

[avocadomayo]

Hey @cpeddecord 👋 I moved applyMetafieldChange back to onChange for the number field. Sorry for the mix-up 🙏