Add source generator

[aljaff94]

Auto assign defined fields to name of field and incremental value start from 1
class must be defined as partial with Attribute [SmartEnumGenerator] and all fields must be static

example class with SmartEnumGenerator and no inheritance from SmartEnum

[SmartEnumGenerator]
public sealed partial class Permissions
{
    public static readonly Permissions Dashboard;
    public static readonly Permissions UserManagement;
}

generated code will be

namespace SmartEnum.SourceGenerator.UnitTests
{
    public sealed partial class Permissions  : SmartEnum <Permissions> 
    {
        public Permissions(string name, int value) : base(name, value) {}

        static Permissions()
        {
	   Dashboard = new Permissions(nameof(Dashboard), 1);
	   UserManagement = new Permissions(nameof(UserManagement), 2);
        }
    }
}

example class with SmartEnumGenerator and inheritanced from SmartEnum

namespace SmartEnum.SourceGenerator.UnitTests
{
    [SmartEnumGenerator]
    public sealed partial class Subscriptions : SmartEnum<Subscriptions>
    {
        public static readonly Subscriptions Free;
        public static readonly Subscriptions Sliver;
        public static readonly Subscriptions Gold;

        public Subscriptions(string name, int value) : base(name, value)
        {
        }
    }
}

generated code will be

namespace SmartEnum.SourceGenerator.UnitTests

{
    public sealed partial class Subscriptions  
    {
        static Subscriptions()
        {
	  Free = new Subscriptions(nameof(Free), 1);
	  Sliver = new Subscriptions(nameof(Sliver), 2);
	  Gold = new Subscriptions(nameof(Gold), 3);
        }
    }
}

Note: //in future we can define a custom attribute to ignore field
I'll do a pull request for this feature in separated package called SmartEnum.SourceGenerator