feat: iOS NucleusFont string initializer

[Priva28]

In order to resolve a NucleusFont from a backend response, the iOS app needs a way to go from a token in a string format to the natively defined default. This PR adds a simple optional initializer to NucleusFont to add that functionality.

let font = NucleusFont(token: "b2") ?? .b4

// world app
Text("Hello, World!")
  .wldFont(font)