feat(foundations): radius implementation

Changes description

Add a VitaminRadius enum to define all allowed corner radiuses as constants

Context

This PR resolves #8 , by providing fixed corner radius for every view

Checklist

[x] Make sure you are requesting to pull a topic/feature/bugfix branch. Please, don't request directly from your main!
[x] Check commits & PR names matches our requested structure. It must follow the https://www.conventionalcommits.org pattern.
[x] Check your code additions will fail neither code linting checks.
[ ] I have reviewed the submitted code.
[x] I have tested on an iPhone device/simulator.
[x] I have tested on an iPad device/simulator.
[ ] If it includes design changes, please ask for a review with a core team designer.

Does this introduce a breaking change?

Screenshots

iPhone

iPad

Other information

I explored many ideas for this foundation, such as a method that would directly modify the corner radius of any view, but that felt a little too invasive to me. I preferred just defining the correct values in an enum, and let the developer use them (every iOS developer should know how to apply a radius to a view ;) ) I also hesitated for the VitaminRadius between two solution :

directly inherit from CGFloat and use the `rawValue'
define a computed property of type CGFloat and use this property

Finallyn the first one felt more intuitive, but i am open to discussion.

Note : After discussion with @lauthieb , I took advantage of this PR to modify colors of labels in the the showcase for shadows, to unify the screens between shadows and radius, and have a render more like the figma one.

Decathlon / vitamin-ios