fix: resolve TreeView deprecation in Unity 6.2+

[chikacc]

Description

Unity 6 (6000.2) has deprecated the non-generic TreeView, TreeViewItem, and TreeViewState classes in UnityEditor.IMGUI.Controls.

• Unity 6000.2+: Triggers obsolete warnings (CS0618).
• Unity 6000.5+: Triggers obsolete compiler errors (CS0619).

Solution

Introduced type aliasing for these classes to their generic counterparts (TreeView<int>, etc.) using the #if UNITY_6000_2_OR_NEWER preprocessor directive.

• Removes deprecation warnings in Unity 6000.2 and later.
• Fixes build errors in Unity 6000.5 and later.

[neuecc]

thank you, I'll release soon.