[Instantiable]: Add generated interface implementations for generic constraints purposes.

[willnationsdev]

I can use the InstantiableAttribute to define various static New() methods for types. However, if I want to write a generic method that takes advantage of these static New() methods, I'd have to provide my own means of doing so.

For example:

public interface INewable;

public interface INewable<T> : INewable
{
    abstract static T New();
}

Then, if people want to customize the names, (which could cause conflicts with other implementing partial declarations), we can just allow the user to override the generated interface name. For example:

// Sample declaration with custom names.
[Instantiable("Init", "CreateFromMilliseconds", GeneratedInterfaceType = "IGodotTemporal"]
public partial class Clock : Control
{
    public void Init(int milliseconds);
}

// Resulting generated interfaces (which `Namespace.Clock.g.cs` would now auto-implement for me).
public interface IGodotTemporal;

public interface IGodotTemporal<T> : IGodotTemporal
{
    void Init(int milliseconds);
    abstract static T CreateFromMilliseconds(int milliseconds);
}

// Usage
var clock = Clock.CreateFromMilliseconds(2000);

To implement this feature, we'd need to...

1. Add the property to the InstantiableAttribute.
2. Extract the value from each one by parsing the attribute text.
3. De-duplicate the type names and consolidate which methods would need to be part of each interface.
4. Ensure the interfaces are added to the RegisterPostInitialiationOutput for the source generator.
   • Or, alternatively, check if such a type has already been defined by a referenced assembly (e.g. if the user purposefully wants to isolate the abstractions from the main project for tests or use in other class libraries, etc.).
   • Or, alternatively, an assembly attribute could "remap" the would-be generated interface to instead use an existing type declaration from a referenced assembly. Something like, [assembly: RemapInstantiableInterface("IGodotTemporal", "MyClassLib.TimeStuff.IGodotTemporal")].
5. The existing generated source would need to be updated to include the interface in its BaseListSyntax.