Add 529 plan contribution deduction to West Virginia state income tax

[MaxGhenis]

Summary

West Virginia 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 WV 529 contribution deduction parameters (caps by filing status)
2. Create a variable for the WV 529 subtraction
3. Integrate into WV AGI subtractions

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

Note: West Virginia 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.