Add superprojects.json and lint workflow for validation

.github/superprojects.json

@@ -0,0 +1,7 @@
+{
+  "usgs-coupled-subtrees/iphreeqc": {
+    "superprojects": [
+      "usgs-coupled-subtrees/webmod"
+    ]
+  }
+}

.github/workflows/lint-superprojects.yml

@@ -0,0 +1,16 @@
+name: Lint superprojects.json
+
+on:
+  pull_request:
+    paths:
+      - '.github/superprojects.json'
+  workflow_call:
+  workflow_dispatch:
+
+jobs:
+  lint-superprojects:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: usgs-coupled-subtrees/sync-subtrees-action/lint-superprojects/@main
+        with:
+          ssh-private-key: ${{ secrets.SSH_PRIVATE_KEY }}