Update eslint (#642)

eslint.config.mjs

@@ -0,0 +1,47 @@
+import js from '@eslint/js';
+import mocha from 'eslint-plugin-mocha';
+import globals from 'globals';
+
+export default [
+  {
+    ignores: [
+      //
+      '**/elm-stuff',
+      '**/fixtures',
+      '**/templates',
+      '**/flow-typed',
+    ],
+  },
+  {
+    rules: {
+      ...js.configs.recommended.rules,
+      'no-inner-declarations': 'off',
+      'no-prototype-builtins': 'off',
+      'no-unused-vars': ['error', { caughtErrorsIgnorePattern: '^_' }],
+    },
+    languageOptions: {
+      globals: {
+        ...globals.node,
+      },
+    },
+  },
+  {
+    files: ['tests/*'],
+    plugins: {
+      mocha,
+    },
+    languageOptions: {
+      globals: {
+        ...globals.mocha,
+      },
+    },
+    rules: {
+      'mocha/handle-done-callback': 'error',
+      'mocha/no-exclusive-tests': 'error',
+      'mocha/no-exports': 'error',
+      'mocha/no-identical-title': 'error',
+      'mocha/no-nested-tests': 'error',
+      'mocha/no-skipped-tests': 'error',
+    },
+  },
+];