Upgrade ESLint to v9

Author: jsynowiec

.eslintignore

@@ -0,0 +1,66 @@
+// @ts-check
+import eslint from '@eslint/js';
+import vitest from '@vitest/eslint-plugin';
+import eslintConfigPrettier from 'eslint-config-prettier';
+import globals from 'globals';
+import tseslint from 'typescript-eslint';
+
+// This is just an example default config for ESLint.
+// You should change it to your needs following the documentation.
+export default tseslint.config(
+  {
+    ignores: ['**/build/**', '**/tmp/**', '**/coverage/**'],
+  },
+  eslint.configs.recommended,
+  eslintConfigPrettier,
+  {
+    extends: [...tseslint.configs.recommended],
+
+    files: ['**/*.ts', '**/*.mts'],
+
+    plugins: {
+      '@typescript-eslint': tseslint.plugin,
+    },
+
+    rules: {
+      '@typescript-eslint/explicit-function-return-type': 'warn',
+    },
+
+    languageOptions: {
+      parser: tseslint.parser,
+      ecmaVersion: 2020,
+      sourceType: 'module',
+
+      globals: {
+        ...globals.node,
+      },
+
+      parserOptions: {
+        project: './tsconfig.json',
+      },
+    },
+  },
+  {
+    files: ['__tests__/**'],
+
+    plugins: {
+      vitest,
+    },
+
+    rules: {
+      ...vitest.configs.recommended.rules,
+    },
+
+    settings: {
+      vitest: {
+        typecheck: true,
+      },
+    },
+
+    languageOptions: {
+      globals: {
+        ...vitest.environments.env.globals,
+      },
+    },
+  },
+);