Add font_design modifier

[supernintendo]

Adds the fontDesign text input and output modifier:

<VStack id="font_design">
  <Text modifiers={font_design(@native, design: :default)}>This text is default</Text>
  <Text modifiers={font_design(@native, design: :monospaced)}>This text is monospaced</Text>
  <Text modifiers={font_design(@native, design: :rounded)}>This text is rounded</Text>
  <Text modifiers={font_design(@native, design: :serif)}>This text is serif</Text>
</VStack>

Closes #300

[carson-katri]

I can look into availability checks in the registry generator.