Convert eslint to flat config

Author: IanVS

.eslintrc.js

@@ -0,0 +1,3 @@
+{
+  "eslint.experimental.useFlatConfig": true
+}

.vscode/settings.json

@@ -0,0 +1,28 @@
+import noOnlyPlugin from "eslint-plugin-no-only-tests";
+import eslint from "@eslint/js";
+
+export default [
+  eslint.configs.recommended,
+  {
+    rules: {
+      semi: "error",
+      "prefer-const": "error",
+    },
+  },
+  {
+    files: ["**/*.js"],
+    languageOptions: {
+      globals: {
+        require: "readonly",
+        module: "readonly",
+      },
+    },
+  },
+  // Prevent .only in tests
+  {
+    plugins: {
+      "no-only-tests": noOnlyPlugin,
+    },
+    rules: { "no-only-tests/no-only-tests": "error" },
+  },
+];

eslint.config.mjs

@@ -12,7 +12,7 @@ function format(css) {
 }
 
 async function run(input, output, opts, postcssOpts = {}) {
-  let result = await postcss([plugin(opts)]).process(input, postcssOpts);
+  const result = await postcss([plugin(opts)]).process(input, postcssOpts);
   expect(format(result.css)).toEqual(format(output));
   expect(result.warnings()).toHaveLength(0);
 }

index.test.mjs

@@ -29,6 +29,7 @@
     "postcss": "^8.3.0"
   },
   "devDependencies": {
+    "@eslint/js": "^8.57.0",
     "c8": "^7.11.3",
     "clean-publish": "^3.4.2",
     "eslint": "^8.57.0",