upgrade eslint v9

eslint.config.js

@@ -0,0 +1,43 @@
+import babelParser from '@babel/eslint-parser';
+import js from '@eslint/js';
+import globals from 'globals';
+import noOnlyTests from 'eslint-plugin-no-only-tests';
+
+export default [
+  {
+    // https://github.com/eslint/eslint/discussions/18304#discussioncomment-9069706
+    ignores: [
+      'node_modules/*',
+      'dist/*',
+      'coverage/*',
+    ],
+  },
+  {
+    languageOptions: {
+      parser: babelParser,
+      parserOptions: {
+        ecmaVersion: 2022,
+        sourceType: 'module',
+        requireConfigFile: false,
+        babelOptions: {
+          plugins: ['@babel/plugin-syntax-import-assertions'],
+        },
+      },
+      globals: {
+        ...globals.browser,
+        ...globals.mocha,
+        ...globals.chai,
+        ...globals.node,
+      },
+    },
+    rules: {
+      ...js.configs.recommended.rules,
+      // turn this off for Prettier
+      'no-irregular-whitespace': 'off',
+      'no-only-tests/no-only-tests': 'error',
+    },
+    plugins: {
+      'no-only-tests': noOnlyTests,
+    },
+  },
+];