feat(postcodes/MH): add Marshall Islands postcodes (#1039) (#1418)

Adds the 2 main Marshall Islands ZIP codes:
  96960 Majuro (capital, Majuro Atoll)
  96970 Ebeye (Kwajalein Atoll)

Uses US ZIP system (Compact of Free Association). Now passes the
cross-reference validator after the MH regex fix in #1414 made the
+4 extension optional.

Refs: #1039
Builds on: #1414

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dr5hn

contributions/postcodes/MH.json

@@ -0,0 +1,22 @@
+[
+  {
+    "code": "96960",
+    "country_id": 137,
+    "country_code": "MH",
+    "state_id": 5656,
+    "state_code": "MAJ",
+    "locality_name": "Majuro",
+    "type": "full",
+    "source": "manual"
+  },
+  {
+    "code": "96970",
+    "country_id": 137,
+    "country_code": "MH",
+    "state_id": 5652,
+    "state_code": "KWA",
+    "locality_name": "Ebeye",
+    "type": "full",
+    "source": "manual"
+  }
+]