tools: Switch to flat config for eslint

Both the .json and .yml config formats are deprecated in favor
of flat configs exported from eslint.config.js files.

The good news is that gjs' config is now available as a shared
config from the `eslint-config-gnome` package in World/javascript,
so we no longer have to copy the entire thing.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/408>

eslint.config.js

@@ -0,0 +1 @@
+tools/eslint.config.js

tools/eslint.config.js

@@ -0,0 +1,37 @@
+// SPDX-FileCopyrightText: 2025 Florian Müllner <fmuellner@gnome.org>
+// SPDX-License-Identifier: MIT OR LGPL-2.0-or-later
+
+import {defineConfig} from '@eslint/config-helpers';
+import gnome from 'eslint-config-gnome';
+
+export default defineConfig([
+    gnome.configs.recommended,
+    gnome.configs.jsdoc,
+    {
+        rules: {
+            camelcase: ['error', {
+                properties: 'never',
+            }],
+            'consistent-return': 'error',
+            'eqeqeq': ['error', 'smart'],
+            'key-spacing': ['error', {
+                mode: 'minimum',
+                beforeColon: false,
+                afterColon: true,
+            }],
+            'prefer-arrow-callback': 'error',
+            'jsdoc/require-param-description': 'off',
+            'jsdoc/require-jsdoc': ['error', {
+                exemptEmptyFunctions: true,
+                publicOnly: {
+                    esm: true,
+                },
+            }],
+        },
+        languageOptions: {
+            globals: {
+                global: 'readonly',
+            },
+        },
+    },
+]);

tools/package.json

@@ -9,6 +9,6 @@
   },
   "dependencies": {
     "ci-run-eslint": "git+https://gitlab.gnome.org/World/javascript/ci-run-eslint.git",
-    "eslint-plugin-jsdoc": "^48.11.0"
+    "eslint-config-gnome": "git+https://gitlab.gnome.org/World/javascript/eslint-config-gnome.git"
   }
 }