Add policyengine-uk dependency

[MaxGhenis]

Summary

Add policyengine-uk as a dependency in pyproject.toml to enable UK tax calculations.

Implementation

• Add policyengine-uk>=2.0.0 to the dependencies list in pyproject.toml
• Ensure compatibility with existing policyengine-us dependency

Acceptance Criteria

• policyengine-uk is importable after installation
• Existing US tests still pass
• No dependency conflicts

Part of #59