chore: migrate from next lint & move to flat config file for ESLint

Author: steadyfall

.eslintrc.json

@@ -1,9 +1,8 @@
+// eslint-disable-next-line @typescript-eslint/no-require-imports
 const path = require('path');
 
 const buildEslintCommand = (filenames) =>
-  `next lint --fix --file ${filenames
-    .map((f) => path.relative(process.cwd(), f))
-    .join(' --file ')}`;
+  `npx eslint --fix ${filenames.map((f) => path.relative(process.cwd(), f)).join(' ')}`;
 
 module.exports = {
   '*.{js,jsx,ts,tsx}': [buildEslintCommand, 'prettier --write'],

.lintstagedrc.js

@@ -0,0 +1,66 @@
+import { defineConfig } from "eslint/config";
+import { fixupConfigRules, fixupPluginRules } from "@eslint/compat";
+import prettier from "eslint-plugin-prettier";
+import _import from "eslint-plugin-import";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import js from "@eslint/js";
+import { FlatCompat } from "@eslint/eslintrc";
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const compat = new FlatCompat({
+    baseDirectory: __dirname,
+    recommendedConfig: js.configs.recommended,
+    allConfig: js.configs.all
+});
+
+export default defineConfig([{
+    extends: fixupConfigRules(compat.extends(
+        "next/core-web-vitals",
+        "next/typescript",
+        "plugin:prettier/recommended",
+        "plugin:import/errors",
+        "plugin:import/warnings",
+        "plugin:import/typescript",
+    )),
+
+    plugins: {
+        prettier: fixupPluginRules(prettier),
+        import: fixupPluginRules(_import),
+    },
+
+    rules: {
+        // Code quality
+        "no-console": "warn",
+        "no-debugger": "error",
+        "no-unused-vars": "off", // Handled by TS version above
+
+        // Import sorting and cleanliness
+        "import/order": ["error", {
+            groups: [
+                "builtin",
+                "external",
+                "internal",
+                "parent",
+                "sibling",
+                "index",
+                "object",
+                "type",
+            ],
+
+            "newlines-between": "always",
+
+            alphabetize: {
+                order: "asc",
+                caseInsensitive: true,
+            },
+        }],
+
+        "import/newline-after-import": "error",
+        "import/no-duplicates": "error",
+        
+        // Prettier takes over formatting rules now
+        "prettier/prettier": "error",
+    },
+}]);