Add skill system with skill tree interface

Introduces a new skill system allowing players to learn and manage skills through a skill tree. Updates `Form1.cs` to include a "Skills" option in the toolbar and menu. Adds `SkillTreeControl` for skill tree UI and modifies `GameEngine` to handle new skill-related commands. Updates `Player` class to track skill points and learned skills. Introduces `SkillTreeForm` for managing the skill tree window. Populates the skill tree with various skills across categories and adjusts the experience system to provide skill points on level up. Enhances the UI with new buttons and labels for skill learning.

Author: ewdlop