Region-based allocation for AST nodes

[SIGMazer]

Description

As the AST grows more complex, managing it becomes increasingly difficult, and free_ast becomes larger. This PR introduces a single Region to allocate all AST nodes, allowing them to be freed at once, simplifying memory management and reducing overhead.

Related Issue

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Refactor

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have documented my changes in the code or documentation
• I have added tests that prove my changes work (if applicable)
• I have run the unit tests locally
• I have run the valgrind memory tests locally
• All new and existing tests pass