Add linter GH action

.github/workflows/linter.yml

@@ -0,0 +1,102 @@
+name: Check for linter warnings / exceptions
+
+on:
+  pull_request_target:
+    branches:
+      - master
+
+jobs:
+  check-duplication:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          ref: ${{ github.event.pull_request.base.sha }}
+
+      - name: Fetch base and target branches
+        run: |
+          git fetch origin +refs/heads/${{ github.event.pull_request.base.ref }}:refs/remotes/origin/${{ github.event.pull_request.base.ref }}
+          git fetch origin +refs/pull/${{ github.event.pull_request.number }}/merge:refs/remotes/pull/${{ github.event.pull_request.number }}/merge
+
+      - name: Get the diff
+        run: git diff --name-only origin/${{ github.event.pull_request.base.ref }}...refs/remotes/pull/${{ github.event.pull_request.number }}/merge > __changed_files.txt
+
+      - name: Run linter on base branch
+        run: npx eslint --no-inline-config --format json $(cat __changed_files.txt) > __base.json
+
+      - name: Check out PR
+        run: git checkout ${{ github.event.pull_request.head.sha }}
+
+      - name: Run linter on PR
+        run: npx eslint --no-inline-config --format json $(cat __changed_files.txt) > __pr.json
+
+      - name: Compare them and post comment if necessary
+        if: env.filtered_report_exists == 'true'
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const fs = require('fs');
+            const path = require('path');
+            const process = require('process');
+            
+            function parse(fn) {
+              return JSON.parse(fs.readFileSync(fn)).reduce((memo, data) => {
+                const file = path.relative(process.cwd(), data.filePath);
+                if (!memo.hasOwnProperty(file)) { memo[file] = { errors: 0, warnings: 0} }
+                data.messages.forEach(({severity}) => {
+                  memo[file][severity > 1 ? 'errors' : 'warnings']++;
+                });
+                return memo;
+              }, {})
+            }
+            
+            function mkDiff(old, new_) {
+              const files = Object.fromEntries(
+                Object.entries(new_)
+                  .map(([file, {errors, warnings}]) => {
+                    const {errors: oldErrors, warnings: oldWarnings} = old[file] || {};
+                    return [file, {errors: errors - (oldErrors ?? 0), warnings: warnings - (oldWarnings ?? 0)}]
+                  })
+                  .filter(([_, {errors, warnings}]) => errors > 0 || warnings > 0)
+              )
+              return Object.values(files).reduce((memo, {warnings, errors}) => {
+                memo.errors += errors;
+                memo.warnings += warnings;
+                return memo;
+              }, {errors: 0, warnings: 0, files})
+            }
+            
+            function mkComment({errors, warnings, files}) {
+              if (errors === 0 && warnings === 0) return;
+              const summary = [];
+              if (errors) summary.push(`**${errors}** linter errors`)
+              if (warnings) summary.push(`**${warnings}** linter warnings`)
+              let cm = `This pr adds ${summary.join(' and ')} (possibly disabled through directives):\n\n`;
+              Object.entries(files).forEach(([file, {errors, warnings}]) => {
+                const summary = [];
+                if (errors) summary.push(`+${errors} errors`);
+                if (warnings) summary.push(`+${warnings} warnings`)
+                cm += ` * \`${file}\` (${summary.join(', ')})\n`
+              })
+              return cm;
+            }
+            
+            const [base, pr] = ['__base.json', '__pr.json'].map(parse);
+            const comment = mkComment(mkDiff(base, pr));
+            
+            if (comment) {
+              github.rest.issues.createComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: context.issue.number,
+                body: comment
+              });
+            }