NY state tax model missing addition modification for post-2018 alimony received

New York State personal income tax rules decoupled from the federal Tax Cuts and Jobs Act (TCJA) treatment of alimony. Under NY Tax Law § 612(b) and Form IT-201 instructions, alimony received under divorce or separation agreements executed after December 31, 2018, remains taxable for New York state tax purposes and must be added back to Federal Adjusted Gross Income (AGI) when calculating New York AGI (`ny_agi`).
Currently in `policyengine-us`, `ny_agi` relies on `ny_additions`, but `ny_additions` has no additions modeled and evaluates to `0`. As a result, post-2018 `alimony_income` is omitted from `ny_agi`.
### Expected Behavior
For tax years 2019 and later, if a tax unit receives `alimony_income`, it should be included in `ny_additions` (or added to `ny_agi`) so that New York Adjusted Gross Income reflects NY State's non-conformity with federal TCJA alimony provisions.
### Actual Behavior
`ny_additions` has no additions modeled:
```python
# policyengine_us/variables/gov/states/ny/tax/income/adjusted_gross_income/ny_additions.py
class ny_additions(Variable):
...
# No additions modeled in PolicyEngine US.
```

Since Federal AGI excludes post-2018 alimony, ny_agi (adjusted_gross_income + ny_additions - ny_agi_subtractions) also excludes it, understating New York taxable income for alimony recipients.

Affected Files:
- policyengine_us/variables/gov/states/ny/tax/income/adjusted_gross_income/ny_agi.py
- policyengine_us/variables/gov/states/ny/tax/income/adjusted_gross_income/ny_additions.py

Statutory & Regulatory References:
- N.Y. Tax Law § 612(b) (Modifications Increasing Federal Adjusted Gross Income)
- NY Form IT-201 / IT-225 Instructions: Alimony received under post-2018 agreements is required to be reported as an addition modification (Addition code A-102 / Line 23 of IT-201).

Start by reading policyengine_us/variables/gov/states/ny/tax/income/adjusted_gross_income/ny_additions.py and ny_agi.py, then review the cited New York tax references. Done means post-2018 alimony_income is included in NY additions or NY AGI for tax years 2019 and later, so ny_agi reflects the required addition modification.

Written by the indexing model from the issue text.