fix(codegen): Adds access modifiers to serializable fields (#633)

- [X] Adds options for SerializableField.

Example:
```cs
[SerializableField(0, getter: "protected", setter: "protected", isVritual: true)]
private int _someField;
```

Defaults: `getter: "public", setter: "public", isVirtual: false`

Projects/SerializationGenerator/SerializerSyntaxReceiver.cs

@@ -13,12 +13,10 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
  *************************************************************************/
 
-using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
-using SourceGeneration;
 
 namespace SerializationGenerator
 {