ci: adds GH action that calculates rule diff

Fixes #266

.github/workflows/rule-diff.yml

@@ -0,0 +1,57 @@
+name: Rule Diff
+
+on:
+  pull_request:
+    branches:
+      - main
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+
+permissions:
+  contents: read
+
+jobs:
+  diff:
+    name: Generate diff
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout the PR source
+        uses: actions/checkout@v4
+        with:
+          path: source
+      - name: Checkout target ref ${{ github.base_ref }}
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.base_ref }}
+          path: target
+      - name: Use Node.js LTS
+        uses: actions/setup-node@v4
+        with:
+          node-version: lts/*
+
+      - name: Determine npm cache directory windows
+        id: npm-cache-dir-windows
+        if: runner.os == 'Windows'
+        run: echo "dir=$(npm config get cache)" >> $env:GITHUB_OUTPUT
+      - name: Determine npm cache directory non-windows
+        id: npm-cache-dir
+        if: runner.os != 'Windows'
+        run: echo "dir=$(npm config get cache)" >> $GITHUB_OUTPUT
+      - uses: actions/cache@v3
+        with:
+          path: ${{ steps.npm-cache-dir-windows.outputs.dir || steps.npm-cache-dir.outputs.dir }}
+          key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json', '**/package.json') }}
+          restore-keys: |
+            ${{ runner.os }}-node-
+
+      - name: install the PR source
+        run: 'npm ci --ignore-scripts --force'
+        working-directory: ./source
+      - name: install the PR target
+        run: 'npm ci --ignore-scripts --force'
+        working-directory: ./target
+
+      - name: run eslint
+        run: compare-eslint-configs .eslintrc ../target/.eslintrc -d -m --verbose-configs >> $GITHUB_STEP_SUMMARY
+        working-directory: ./source

.knip.jsonc

@@ -3,5 +3,5 @@
   "eslint": {
     "config": [".eslintrc", "index.js", "esm.js", "mocha.js"]
   },
-  "ignoreDependencies": ["@types/eslint"]
+  "ignoreDependencies": ["@types/eslint", "compare-eslint-configs"]
 }