Refactor and enhance the AlgEffectus.Core module

[Maokami]

• Updated pretty-printing for Value to use toString for variable names.
• Refactored renaming functions to improve termination proofs and ensure proper handling of names.
• Introduced scoped notation for single and multi-step reductions in semantics.
• Improved substitution functions to ensure proper handling of alpha-renaming and termination.
• Enhanced syntax definitions with size calculations for values, computations, and handlers.
• Added a new TypeSafety module to formalize preservation and progress theorems for the type system.
• Introduced a Typing module to define the core type system for algebraic effects and handlers.
• Improved overall structure and readability of the codebase.