Upgrade ESLint to v9 with flat config format (#322)

Migrate from ESLint 8 to ESLint 9, which requires converting the legacy
.eslintrc.json configuration to the new flat config format (eslint.config.js).

Changes:
- Update eslint to ^9.17.0
- Add @eslint/js, @eslint/compat, and globals packages
- Create eslint.config.js with flat config format
- Remove .eslintrc.json and .eslintignore (now handled in config)
- Update lint scripts to remove deprecated --ext flag
- Fix unused catch binding warnings using ES2019+ optional catch binding

Author: olsh

.eslintignore

@@ -0,0 +1,96 @@
+const js = require("@eslint/js");
+const globals = require("globals");
+const { fixupPluginRules } = require("@eslint/compat");
+const jqueryPlugin = require("eslint-plugin-jquery");
+
+module.exports = [
+    // Ignore patterns
+    {
+        ignores: [
+            "node_modules/**",
+            "build/**",
+            "test-results/**",
+            "logos/**",
+            "translations/**",
+            "src/styles/**",
+            "src/images/**",
+            "src/_locales/**",
+            "src/sound/**"
+        ]
+    },
+
+    // Base configuration for all JS files
+    {
+        files: ["**/*.js"],
+        ...js.configs.recommended,
+        plugins: {
+            jquery: fixupPluginRules(jqueryPlugin)
+        },
+        languageOptions: {
+            ecmaVersion: 2021,
+            sourceType: "script",
+            globals: {
+                ...globals.browser,
+                ...globals.es2021,
+                browser: "readonly",
+                chrome: "readonly",
+                $: "readonly",
+                jQuery: "readonly",
+                Mustache: "readonly",
+                DOMPurify: "readonly",
+                timeago: "readonly",
+                importScripts: "readonly"
+            }
+        },
+        rules: {
+            indent: ["error", 4, { SwitchCase: 1 }],
+            quotes: ["error", "double", { avoidEscape: true }],
+            semi: ["error", "always"],
+            curly: ["error", "all"],
+            "no-console": "off",
+            "no-unused-vars": ["warn", { args: "none" }],
+            "no-var": "off",
+            "no-empty": ["warn", { allowEmptyCatch: false }],
+            "jquery/no-ready": "warn"
+        }
+    },
+
+    // Background script overrides
+    {
+        files: ["src/scripts/background.js"],
+        languageOptions: {
+            globals: {
+                appGlobal: "readonly",
+                getFeeds: "readonly",
+                getSavedFeeds: "readonly",
+                markAsRead: "readonly",
+                toggleSavedFeed: "readonly",
+                openFeedlyTab: "readonly",
+                resetCounter: "readonly",
+                getAccessToken: "readonly",
+                readOptions: "readonly",
+                initialize: "readonly"
+            }
+        }
+    },
+
+    // Gruntfile configuration
+    {
+        files: ["Gruntfile.js"],
+        languageOptions: {
+            globals: {
+                ...globals.node
+            }
+        }
+    },
+
+    // Core and options scripts
+    {
+        files: ["src/scripts/core.js", "src/scripts/options.js"],
+        languageOptions: {
+            globals: {
+                FeedlyApiClient: "readonly"
+            }
+        }
+    }
+];