fix(cocoapods): fix to pass cocoapods podspec validation

daniel-dumortier commented 2 years ago

Before pushing to cocoapods, we need to run pod lib lint And this enlightened that.... the project does not compile if not in SPM context (typically, for Cocoapods)

To fix that, I had to :

change the SwiftGen configuration for Fonts and Icons
add the swift version in podspec (5.3)

The validation now passes, so once this PR merged, I will be able to push it to Cocoapods trunk repo

After discussion with @lauthieb , I also modified the README to encourage users to use an exact version of Vitamin in their project (with SPM and Cocoapods)