Support for nested models

[dominik-hadl]

Right now, it's not possible to generate code for this selection:

struct OuterModel {
    var name = ""
    var id = 0

    struct InnerModel {
        var someProperty = ""
    }
}

This will throw an exception in model-generator when generating the code and Model Boiler will present a failure notification.

---

Expected behaviour: Models should be split up by Model Boiler and then passed one by one into model-generator, while keeping the module name (= outer struct/class name).

All the generated code would then be merged into one string and that will be put back into clipboard and the resulting code would look like this:

extension OuterModel: Serializable {
    // boilerplate code here
}

extension OuterModel.InnerModel: Serializable {
    // boilerplate code here
}