feat: support flat config format default for ESLint 9 (#277)

* feat: export FlatConfig to support ESLint v9

* refactor: add backwards compatibility for ESLint < 9 and extract common configuration code

* chore: updated README.md

* fix: adjust plugin flowtype in node.factory.js to work with flat config properly

* fix(test): created configuration for tests to be run both with ESLint v8 & v9

* fix: maintain previous order of plugins in final configuration

* fix: bring back compatibility with any jest version in test package

* refactor: export named functions from .factory.js source files

Author: artus9033

Diff for: README.md

@@ -1,6 +1,6 @@
 # @callstack/eslint-config
 
-Callstack ESLint config for React Native, React and Node.js projects, utilizing Flow, TypeScript, Prettier and Jest with sensible defaults.
+Callstack ESLint config for React Native, React and Node.js projects, utilizing Flow, TypeScript, Prettier and Jest with sensible defaults. Supports both eslintrc and flat config.
 
 ## Installation
 
@@ -18,18 +18,40 @@ npm install --save-dev eslint @callstack/eslint-config
 
 ## Usage
 
-You can choose one of the following environments to work with by extending your ESLint config (`.eslintrc`, or `eslintConfig` field in `package.json`) with `@callstack` config tailored to your project.
+You can choose one of the following environments to work with by extending your ESLint config (`eslint.config.mjs` for flat config, or `.eslintrc` / `eslintConfig` field in `package.json` for the eslintrc config style) with `@callstack` config tailored to your project.
 
 ### React Native config
 
 Usage:
 
+#### eslintrc format (ESLint < v9)
+
 ```json
 {
   "extends": "@callstack"
 }
 ```
 
+#### flat config format (`eslint.config.mjs`, ESLint 9+)
+
+```js
+import callstackConfig from '@callstack/eslint-config/react-native.flat.js';
+
+export default [
+  {
+    ignores: [
+      // ignored files go here
+    ],
+  },
+  ...callstackConfig,
+  {
+    rules: {
+      // your custom rules
+    },
+  },
+];
+```
+
 Plugins used:
 
 - **React config**
@@ -42,12 +64,34 @@ Additionally, it sets `"react-native/react-native"` environment and native platf
 
 Usage:
 
+#### eslintrc format (ESLint < v9)
+
 ```json
 {
   "extends": "@callstack/eslint-config/react"
 }
 ```
 
+#### flat config format (`eslint.config.mjs`, ESLint 9+)
+
+```js
+import callstackConfigReact from '@callstack/eslint-config/react.flat.js';
+
+export default [
+  {
+    ignores: [
+      // ignored files go here
+    ],
+  },
+  ...callstackConfigReact,
+  {
+    rules: {
+      // your custom rules
+    },
+  },
+];
+```
+
 Plugins used:
 
 - **Node config**
@@ -58,12 +102,34 @@ Plugins used:
 
 Usage:
 
+#### eslintrc format (ESLint < v9)
+
 ```json
 {
   "extends": "@callstack/eslint-config/node"
 }
 ```
 
+#### flat config format (`eslint.config.mjs`, ESLint 9+)
+
+```js
+import callstackConfigNode from '@callstack/eslint-config/node.flat.js';
+
+export default [
+  {
+    ignores: [
+      // ignored files go here
+    ],
+  },
+  ...callstackConfigNode,
+  {
+    rules: {
+      // your custom rules
+    },
+  },
+];
+```
+
 Plugins used:
 
 - [eslint-config-prettier](https://yarnpkg.com/en/package/eslint-config-prettier)
@@ -79,6 +145,8 @@ Additionally, it sets `es6` and `node` environments.
 
 ### Example of extending the configuration
 
+##### eslintrc format (ESLint < v9)
+
 ```json
 {
   "extends": "@callstack",
@@ -89,6 +157,22 @@ Additionally, it sets `es6` and `node` environments.
 }
 ```
 
+##### flat config format (`eslint.config.mjs`, ESLint 9+)
+
+```js
+import callstackConfig from '@callstack/eslint-config/react-native.flat.js';
+
+export default [
+  ...callstackConfig,
+  {
+    rules: {
+      'global-require': 0,
+      'prefer-destructuring': 0,
+    },
+  },
+];
+```
+
 ### TypeScript
 
 TypeScript is supported out-of-the-box, including importing JS files from TS files and vice-versa. All you need to do is to make sure you have [`typescript`](https://yarnpkg.com/en/package/typescript) module installed.
@@ -103,6 +187,8 @@ yarn eslint --ext '.js,.ts' ./src
 
 To do so, you'll need to override our setup for TS files in your ESLint config:
 
+##### eslintrc format (ESLint < v9)
+
 ```json
 {
   "overrides": [
@@ -116,6 +202,26 @@ To do so, you'll need to override our setup for TS files in your ESLint config:
 }
 ```
 
+##### flat config format (`eslint.config.mjs`, ESLint 9+)
+
+In the flat config, just append another configuration object to the array and be sure to import the `/react-native.flat` file:
+
+```js
+import callstackConfig from '@callstack/eslint-config/react-native.flat.js';
+import tsEslintParser from '@typescript-eslint/parser';
+
+export default [
+  ...callstackConfig,
+  {
+    files: ['**/*.ts', '**/*.tsx'],
+    languageOptions: {
+      parser: tsEslintParser,
+      parserOptions: { project: './packages/**/tsconfig.json' },
+    },
+  },
+];
+```
+
 #### VSCode
 
 If you're VSCode user, you may find adding this config to your `.vscode/settings.json` helpful: