Refactor ny_agi_subtractions

[MaxGhenis]

Currently we use + to sum up the four elements. Instead let's make the pension exclusion its own variable and then use a list parameter for this, to align more with other states and parameterize more of the code.

https://github.com/PolicyEngine/policyengine-us/blob/master/policyengine_us/variables/gov/states/ny/tax/income/adjusted_gross_income/ny_agi_subtractions.py