Add a generator for custom code

[daniel-kuon]

Adds custom code generator that returns a list of RtNodes. This is helpful when you need to generate classes/interfaces that depend on C# types but have specific characteristics in TypeScript that are not needed on the server side.

These generators can generate pretty much every type of TS. Classes/Interface like the library is already able to do but also objects that are generated from these generated types.

Usage Example: Localization

I have different C# DTO classes that represent form data and are used to automatically generate forms via TypeScript. The labels for the form fields are localizable.

public class Login
{
    public string Username { get; set; }
    public string Password { get; set; }
    public bool RememberMe { get; set; }
}

public class Register
{
    public string Username { get; set; }
    public string Password { get; set; }
    public string PasswordRepeat { get; set; }
}

A CustomCodeGenerator then generates a TS interface that combines localization fields for all properties of the two classes. This interface is not needed at all on the server and it would need to be maintained manually whenever one of the C# form classes is changed.

interface ILocales {
    forms: IFormLocales
}

interface IFormLocales {
    username: string;
    password: string;
    rememberMe: string;
    passwordRepeat: string;
}

I use a localization framework that is initialized with an IFormLocales object and when I need a localized string I pass the path to the property of the object (e.g. translate("forms.userName")). To avoid typing the string every time my custom generator also generates a helper object for this:

export const locales: ILocales = {
    forms: {
        password: "forms.password",
        passwordRepeat: "forms.passwordRepeat",
        rememberMe: "forms.rememberMe",
        username: "forms.username"
    }
};

[ruler501]

Should probably add some tests to make this more maintainable going forward. Otherwise maybe update one of the samples to show a complete example of how this could be used in a more advanced context. It is definitely possible that sufficient tests could show that as well though.
Definitely seems like an interesting concept to add, even if I'm still not quite sure what it does. This also reminds me I need to fix the merge errors in my pr and add some tests.

[pavel-b-novikov]

I ageree with @ruler501 - tests needed