[deps] Upgrade eslint to v9 and fix config

Upgrade eslint to v9 - requires upgrading to the new config mechanism "flat
config".

- Switch from .cjs to .js which implies mjs
- Change style of plugins and shared configs (install @eslint/js)

go/frontend/eslint.config.js

@@ -0,0 +1,35 @@
+import globals from "globals";
+import { default as reactPlugin } from "eslint-plugin-react";
+import js from "@eslint/js";
+
+export default [
+  {
+    ...js.configs.recommended,
+    ...reactPlugin.configs.flat.recommended, // This is not a plugin object, but a shareable config object
+    ...reactPlugin.configs.flat["jsx-runtime"],
+    languageOptions: {
+      ecmaVersion: "latest",
+      sourceType: "module",
+      globals: {
+        ...globals.browser,
+      },
+      parserOptions: {
+        ecmaFeatures: {
+          jsx: true,
+        },
+      },
+    },
+    files: ["src/**/*.{js,jsx}"],
+    plugins: {
+      reactPlugin,
+    },
+    settings: {
+      react: {
+        version: "detect",
+      },
+    },
+    rules: {
+      "react/prop-types": [0],
+    },
+  },
+];

go/frontend/package.json

@@ -7,16 +7,17 @@
   "scripts": {
     "build": "vite build",
     "format": "prettier --write src index.html",
-    "lint": "eslint src --ext .jsx --ext .js",
+    "lint": "eslint",
     "start": "vite serve",
     "preview": "vite preview",
     "test": "echo \"Error: no test specified\" && exit 1"
   },
   "author": "Andy O'Neill <[email protected]>",
   "license": "ISC",
   "devDependencies": {
-    "eslint": "^8",
-    "eslint-plugin-react": "^7",
+    "@eslint/js": "^9.12.0",
+    "eslint": "^9",
+    "eslint-plugin-react": "^7.37.1",
     "prettier": "^3",
     "vite": "^5"
   },

go/frontend/vite.config.js

@@ -1,4 +1,3 @@
-/* eslint-disable no-undef */
 import { defineConfig } from "vite";
 import { resolve } from "node:path";