Add UI Toolkit support

[AlexanderVorobyov]

• Update dependencies: com.ugf.editortools to 3.0.0-preview.13, com.ugf.runtimetools to 3.0.0-preview.18, add com.unity.modules.uielements of 1.0.0, and remove of com.unity.modules.imgui.
• Add UI Toolkit implementation.
• Remove IMGUI implementation.

[AlexanderVorobyov]

Create branch from main

issue-26-add-ui-toolkit-support

[AlexanderVorobyov]

Related to:

• https://docs.unity3d.com/Manual/UIE-USS-UnityVariables.html