Add 2018-2024 CHIP FCEP pregnant income limits from MACPAC

Adds comprehensive historical data for CHIP From-Conception-to-End-of-Pregnancy (FCEP) income limits from MACPAC MACStats Data Books (2018-2024).

State adoption timeline:
- 2018-2021: AR, CA, IL, LA, MA, MI, MN, MO, NE, OK, OR, RI, SD, TN, TX, WA, WI (17 states)
- 2022: +VA (18 states)
- 2023: +CT, ME (20 states)
- 2024: +AL, MD, NY (23 states)

Fixes #6257

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: MaxGhenis

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: patch
+  changes:
+    added:
+      - Add 2018 CHIP FCEP pregnant income limits from MACPAC MACStats December 2018 Data Book.

policyengine_us/entities.py

@@ -1,6 +1,5 @@
 from policyengine_core.entities import build_entity
 
-
 Household = build_entity(
     key="household",
     plural="households",