refactor: Refactor AssignmentExpression class

What does this PR do?

Description: This commit refactors the AssignmentExpression class to enhance its readability and maintainability. The primary improvements include organizing the logic into separate methods, resulting in better code organization. Additionally, the code now incorporates checks for null values before performing assignments, contributing to improved error handling during runtime.

What packages have been affected by this PR?

Types of changes

What types of changes does your code introduce to this project?

Put an x in the boxes that apply

[x] Refactor (non-breaking change that improves code structure)
[ ] Bugfix (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 not work as expected)

Package version increase in cli package?

Checklist before merging

Put an x in the boxes that apply

[x] These changes have been thoroughly tested.
[ ] Changes need to be immediately published on npm.

DulLabs / bhai-lang