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.es5.js

@@ -0,0 +1,18 @@
+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
+  },
+};