test(fixtures): add eslint flat config so eslint-direct probe passes

eslint v10 requires eslint.config.js (flat config) — previously the
node-formatter fixture had no config, so eslint-direct's lint-text
call returned 500. Adds a minimal flat config + type:module on
package.json so the ESM config loads.

Author: Blanquitoh

fixtures/node-formatter/eslint.config.js

@@ -0,0 +1,8 @@
+export default [
+  {
+    files: ["**/*.ts", "**/*.js"],
+    rules: {
+      "no-unused-vars": "warn",
+    },
+  },
+];

fixtures/node-formatter/package.json

@@ -2,5 +2,6 @@
   "name": "node-formatter-fixture",
   "version": "0.0.0",
   "private": true,
+  "type": "module",
   "description": "smoke fixture for prettier-direct + eslint-direct"
 }