Add 529 plan contribution deduction to South Carolina state income tax

[MaxGhenis]

Summary

South Carolina offers a state income tax deduction for contributions to 529 education savings plans. This is not currently implemented in PolicyEngine.

What needs to happen

1. Add SC 529 contribution deduction parameters (caps by filing status)
2. Create a variable for the SC 529 subtraction
3. Integrate into SC AGI subtractions

The existing person-level variable investment_in_529_plan can be used as the input.

Note: South Carolina allows a 100% deduction with no cap.

Context

Flagged during a meeting with Matt Comey (JCT) and Dan Feenberg (NBER/TAXSIM) as a gap in state tax calculators.