Rewrite .eslintrc.es5.js to new config format

[MAILPOET-5015]

packages/js/eslint-config/.eslintrc.es5.js

@@ -1,18 +0,0 @@
-module.exports = {
-  extends: ['airbnb/legacy', 'prettier'],
-  env: {
-    amd: true,
-    browser: true,
-  },
-  plugins: ['eslint-plugin-import'],
-  parser: '@babel/eslint-parser',
-  parserOptions: {
-    ecmaVersion: 6,
-    sourceType: 'module',
-  },
-  rules: {
-    'import/prefer-default-export': 0, // we want to stop using default exports and start using named exports
-    'no-underscore-dangle': 0, // Backbone uses underscores, we cannot remove them
-    'import/no-default-export': 1, // no default exports
-  },
-};

packages/js/eslint-config/eslint-es5.config.js

@@ -0,0 +1,36 @@
+const babelParser = require('@babel/eslint-parser');
+const FlatCompat = require('@eslint/eslintrc').FlatCompat;
+const airbnbLegacyConfig = require('eslint-config-airbnb/legacy');
+const prettierConfig = require('eslint-config-prettier');
+const importPlugin = require('eslint-plugin-import');
+const globals = require('globals');
+
+// compat configs
+const compat = new FlatCompat({ baseDirectory: __dirname });
+const airbnbLegacyCompatConfig = compat.config(airbnbLegacyConfig);
+const prettierCompatConfig = compat.config(prettierConfig);
+
+module.exports = [
+  ...airbnbLegacyCompatConfig,
+  ...prettierCompatConfig,
+  {
+    languageOptions: {
+      parser: babelParser,
+      parserOptions: {
+        ecmaVersion: 6,
+        sourceType: 'module',
+      },
+      globals: {
+        ...globals.browser,
+      },
+    },
+    plugins: {
+      import: importPlugin,
+    },
+    rules: {
+      'import/prefer-default-export': 0, // we want to stop using default exports and start using named exports
+      'no-underscore-dangle': 0, // Backbone uses underscores, we cannot remove them
+      'import/no-default-export': 1, // no default exports
+    },
+  },
+];