improve: move to src

src/ClassSyntaxReceiver.cs

@@ -0,0 +1,18 @@
+using System.Collections.Generic;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+
+namespace Polonium.Generators;
+
+public class ClassSyntaxReceiver : ISyntaxReceiver
+{
+    public List<ClassDeclarationSyntax> Classes { get; set; } = new ();
+
+    public void OnVisitSyntaxNode(SyntaxNode syntaxNode)
+    {
+        if (syntaxNode is ClassDeclarationSyntax classDeclaration)
+        {
+            Classes.Add(classDeclaration);
+        }
+    }
+}