feat(codegen): Adds support for partial opt-in with already existing properties (#617)

- [X] Fixes an issue with ordering of properties in serialization.
- [X] Adds opt-in with existing properties.

Example:
```cs
private int _myExistingField;

[SerializableField(1)]
public int MyExistingProperty
{
    get => _myExistingField;
    set
    {
        if (value == 0)
        {
            Parent = null;
        }
        
        if (value != _myExistingField)
        {
            ((ISerializable)this).MarkDirty();
            _myExistingField = value;
        }
    }
}
```

Projects/SerializationGenerator/SourceGeneration/SourceGeneration.Namespace.cs

@@ -23,19 +23,6 @@ namespace SerializationGenerator
 {
     public static partial class SourceGeneration
     {
-        public static void GenerateUsings(this StringBuilder source, IImmutableList<ITypeSymbol> typesUsed)
-        {
-            var enumerable = typesUsed
-                .Select(t => t.ContainingNamespace.Name)
-                .Distinct()
-                .OrderByDescending(t => t);
-
-            foreach (var t in enumerable)
-            {
-                source.Insert(0, $"using {t}{Environment.NewLine}");
-            }
-        }
-
         public static void GenerateNamespaceStart(this StringBuilder source, string namespaceName)
         {
             source.AppendLine($@"namespace {namespaceName}