Upgrade to ESLint 9 with flat config to fix release build

The release build failed with:

    [eslint] Couldn't find FlatESLint, you might need to set eslintPath
    to 'eslint/use-at-your-own-risk'

eslint-webpack-plugin v5 defaults to ESLint's flat config system, which
requires ESLint 9. The project was still on ESLint 8 and had no ESLint
configuration file at all, so linting never effectively ran.

Following the same approach already used in Stepup-Gateway:

- Upgrade eslint to ^9 and add @eslint/js and typescript-eslint
- Drop eslint-config-airbnb and eslint-plugin-import (unused: ESLint 8
  config files never existed, and airbnb does not support flat config)
- Add eslint.config.mjs with recommended + type-checked TS rules
- Scope the webpack ESLint plugin to assets/typescript and fail the
  production build on lint errors
- Fix the two lint findings this surfaced in app.ts and
  registration-print.ts

Author: kayjoosten

assets/typescript/app.ts

@@ -1,3 +1,3 @@
 import jQuery from 'jquery';
-(window as any).jQuery = jQuery;
+(window as unknown as { jQuery: typeof jQuery }).jQuery = jQuery;
 import 'bootstrap';

assets/typescript/registration-print.ts

@@ -15,7 +15,7 @@ import jQuery from 'jquery';
  * limitations under the License.
  */
 
-document.addEventListener('DOMContentLoaded', (_) => {
+document.addEventListener('DOMContentLoaded', () => {
   jQuery('a.registration-print').on('click', (e) => {
     e.preventDefault();
 

eslint.config.mjs

@@ -0,0 +1,35 @@
+import js from '@eslint/js';
+import typescriptEslint from 'typescript-eslint';
+
+export default [
+    {
+        ignores: [
+            'vendor/**',
+            'node_modules/**',
+            'var/**',
+            'public/build/**',
+            'build/**',
+            'dist/**',
+        ],
+    },
+    js.configs.recommended,
+    ...typescriptEslint.configs.recommendedTypeChecked,
+    {
+        files: ['**/*.ts', '**/*.tsx'],
+        languageOptions: {
+            parserOptions: {
+                project: './tsconfig.json',
+                tsconfigRootDir: import.meta.dirname,
+            },
+        },
+        rules: {
+            '@typescript-eslint/no-explicit-any': 'warn',
+            '@typescript-eslint/no-unused-vars': 'warn',
+        },
+    },
+    {
+        // Disable type-checking rules for JS files
+        files: ['**/*.js', '**/*.jsx'],
+        ...typescriptEslint.configs.disableTypeChecked,
+    },
+];

package.json

@@ -3,16 +3,16 @@
     "devDependencies": {
         "@babel/core": "^7.29.7",
         "@babel/preset-env": "^7.29.7",
+        "@eslint/js": "^9.39.4",
         "@symfony/webpack-encore": "^5.0.0",
         "@types/jest": "^30",
         "@types/jquery": "^3.5.32",
         "@types/node": "^20",
         "compass-mixins": "^0.12.10",
         "css-loader": "^7.1.4",
-        "eslint": "^8.57.0",
-        "eslint-config-airbnb": "^19.0",
-        "eslint-plugin-import": "^2.31.0",
+        "eslint": "^9.39.4",
         "eslint-webpack-plugin": "^5.0.3",
+        "typescript-eslint": "^8.59.3",
         "file-loader": "^6.0.0",
         "jest": "^30",
         "less": "^4.6.4",

webpack.config.js

@@ -37,7 +37,10 @@ Encore
     })
     .addLoader({test: /\.scss$/, loader: 'webpack-import-glob-loader', enforce: 'pre'})
     .addPlugin(new ESLintPlugin({
-        extensions: ['js', 'jsx', 'ts', 'tsx', 'vue'],
+        extensions: ['ts', 'js'],
+        files: 'assets/typescript',
+        emitWarning: true,
+        failOnError: Encore.isProduction(),
     }))
     .enableSingleRuntimeChunk()
     .enableSourceMaps(!Encore.isProduction())