Use JS ESLint configs instead of JSON

This is to avoid ERR_IMPORT_ASSERTION_TYPE_MISSING on recent Node.js with updated ESLint.
Fixes errors like the following: ".eslintrc.es6.json" needs an import assertion of type "json"

[MAILPOET-5061]

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

@@ -0,0 +1,20 @@
+module.exports = {
+  extends: ['airbnb/legacy', 'prettier'],
+  env: {
+    amd: true,
+    mocha: true,
+  },
+  parser: '@babel/eslint-parser',
+  parserOptions: {
+    ecmaVersion: 6,
+    sourceType: 'module',
+  },
+  rules: {
+    'no-only-tests/no-only-tests': 2,
+    // Exceptions
+    'func-names': 0,
+    // Temporary
+    'no-underscore-dangle': 0,
+  },
+  plugins: ['no-only-tests'],
+};