chore: add hrynco common library solution

- add the standalone HrynCo.Common solution and projects
- include the shared common library source and tests
- add package metadata and a repo gitignore

HrynCo.Common/Tree/ITreeNode.cs

@@ -0,0 +1,8 @@
+namespace HrynCo.Common.Tree;
+
+public interface ITreeNode<TKey>
+    where TKey : struct
+{
+    TKey Id { get; }
+    TKey? ParentId { get; }
+}