Add a workflow to check versions in the modpack index

See #5.

.github/workflows/check-versions.yaml

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+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: Louis Moureaux <[email protected]>
+
+name: Check versions
+on:
+  push: {}
+  pull_request: {}
+
+jobs:
+  check:
+    name: Check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+      - run: sudo apt-get install curl jq
+      - run: sh check-versions.sh
+

check-versions.sh

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+#!/bin/sh
+
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText:  Tobias Rehbein <[email protected]>
+
+# Checks that the version numbers in the index file match those in the
+# modpacks.
+
+IFS='|'
+jq -r '.modpacks[] | "\(.name)|\(.version)|\(.url)"' index.json \
+	| while read NAME VERSION URL; do
+		RVERSION=$({ if $(echo "$URL" | grep -q '^http'); then
+			curl -s "$URL"
+		else
+			cat "$URL"
+		fi } | jq -r '.info.version')
+
+		[ "$VERSION" != "$RVERSION" ] \
+			&& echo "Version mismatch for \"$NAME\": index $VERSION; remote $RVERSION"
+	done
+