Upgrade ESLint to V9

.changeset/sweet-tables-worry.md

@@ -0,0 +1,9 @@
+---
+'eslint-plugin-primer-react': minor
+---
+
+Add support for ESLint v9
+
+- Adds flat config support
+- Adds meta information to config object
+- Backwards compatibility with v8

.markdownlint-cli2.cjs

@@ -16,11 +16,11 @@ const options = githubMarkdownOpinions.init({
   'no-hard-tabs': false,
   'first-line-heading': false,
   'no-space-in-emphasis': false,
-  'blanks-around-fences': false
+  'blanks-around-fences': false,
 })
 
 module.exports = {
   config: options,
   customRules: ['@github/markdownlint-github'],
-  outputFormatters: [['markdownlint-cli2-formatter-pretty', {appendLink: true}]]
+  outputFormatters: [['markdownlint-cli2-formatter-pretty', {appendLink: true}]],
 }

eslint.config.js

@@ -0,0 +1,52 @@
+const eslintJs = require('@eslint/js')
+const github = require('eslint-plugin-github')
+const globals = require('globals')
+const pluginJest = require('eslint-plugin-jest')
+
+module.exports = [
+  github.getFlatConfigs().recommended,
+  eslintJs.configs.recommended,
+  {
+    languageOptions: {
+      parserOptions: {
+        ecmaVersion: 'latest',
+      },
+      globals: {
+        ...globals.commonjs,
+        ...globals.node,
+      },
+    },
+    rules: {
+      'importPlugin/no-commonjs': 'off',
+      'no-shadow': 'off',
+      'no-unused-vars': [
+        'error',
+        {
+          varsIgnorePattern: '^_',
+        },
+      ],
+    },
+    ignores: ['**/dist/**', '**/node_modules/**'],
+  },
+  {
+    files: ['**/*.test.js'],
+    languageOptions: {
+      globals: {
+        ...pluginJest.environments.globals.globals,
+      },
+    },
+  },
+  {
+    files: ['.eslint.config.js', '.markdownlint-cli2.cjs'],
+    rules: {
+      'filenames/match-regex': 'off',
+      'github/filenames-match-regex': 'off',
+    },
+  },
+  {
+    files: ['src/url.js', 'src/rules/new-color-css-vars.js'],
+    rules: {
+      'importPlugin/extensions': 'off',
+    },
+  },
+]