chore: switch to eslint 9

Update ESLint to version 9 and migrate to flat config format.

* **Add `eslint.config.mjs`**: Create a new file `eslint.config.mjs` with the flat config format for ESLint, including necessary plugins, parser options, rules, and ignore patterns.
* **Remove `.eslintrc.js`**: Delete the file `.eslintrc.js`.
* **Remove `.eslintignore`**: Delete the file `.eslintignore`.
* **Update `package.json`**:
  - Change ESLint version to `^9.0.0` in `devDependencies`.
  - Update `lint` and `format` scripts to use the new ESLint configuration.

---

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eslint.config.mjs

@@ -0,0 +1,81 @@
+import { FlatCompat } from '@eslint/eslintrc';
+import js from '@eslint/js';
+import typescriptParser from '@typescript-eslint/parser';
+import typescriptPlugin from '@typescript-eslint/eslint-plugin';
+import prettierPlugin from 'eslint-plugin-prettier';
+import jestPlugin from 'eslint-plugin-jest';
+import prettierConfig from 'eslint-config-prettier';
+
+const compat = new FlatCompat();
+
+export default [
+  js.configs.recommended,
+  jestPlugin.configs.recommended,
+  jestPlugin.configs.style,
+  typescriptPlugin.configs.recommended,
+  prettierConfig,
+  {
+    files: ['*.ts'],
+    ignores: [
+      '.eslintrc.js',
+      'babel.config.js',
+      'prettier.config.js',
+      'jest.config.js',
+      'rollup.config.js',
+      'build',
+      'build-es5',
+      'dist',
+      'coverage',
+      'types'
+    ],
+    languageOptions: {
+      parser: typescriptParser,
+      parserOptions: {
+        project: 'tsconfig.json',
+        ecmaVersion: 2020,
+        sourceType: 'module'
+      }
+    },
+    plugins: {
+      '@typescript-eslint': typescriptPlugin,
+      prettier: prettierPlugin,
+      jest: jestPlugin
+    },
+    rules: {
+      'prettier/prettier': 'warn',
+      '@typescript-eslint/explicit-function-return-type': 'off',
+      '@typescript-eslint/ban-types': 'warn',
+      '@typescript-eslint/no-explicit-any': 'warn',
+      '@typescript-eslint/prefer-for-of': 'error',
+      '@typescript-eslint/no-for-in-array': 'error',
+      '@typescript-eslint/no-require-imports': 'error',
+      '@typescript-eslint/no-parameter-properties': 'off',
+      '@typescript-eslint/explicit-member-accessibility': 'off',
+      'no-unused-vars': 'off',
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        {
+          vars: 'all',
+          args: 'after-used',
+          ignoreRestSiblings: true
+        }
+      ],
+      'no-shadow': 'off',
+      '@typescript-eslint/no-shadow': 'error',
+      '@typescript-eslint/no-object-literal-type-assertion': 'off',
+      '@typescript-eslint/explicit-module-boundary-types': 'off',
+      '@typescript-eslint/no-namespace': 'error',
+      'jest/no-conditional-expect': 'off',
+      'linebreak-style': ['error', 'unix'],
+      'no-irregular-whitespace': ['error', { skipComments: true }],
+      'no-alert': 'error',
+      'prefer-const': 'error',
+      'no-return-assign': 'error',
+      'no-useless-call': 'error',
+      'no-useless-concat': 'error',
+      'prefer-template': 'error',
+      'no-undef': 'off',
+      'no-unreachable': 'off'
+    }
+  }
+];

package.json

@@ -77,7 +77,7 @@
     "@typescript-eslint/parser": "^6.3.0",
     "browser-sync": "^3.0.2",
     "concurrently": "^8.2.0",
-    "eslint": "^8.46.0",
+    "eslint": "^9.0.0",
     "eslint-config-prettier": "^9.0.0",
     "eslint-plugin-jest": "^28.5.0",
     "eslint-plugin-prettier": "^5.0.0",