package.json: document the required nodejs version

To avoid issues about not being able to run make / npm install. This
sadly only produces a warning, but that hint might be good enough.

```
$ npm install
npm WARN EBADENGINE Unsupported engine {
npm WARN EBADENGINE   package: undefined,
npm WARN EBADENGINE   required: { node: '>= 18' },
npm WARN EBADENGINE   current: { node: 'v16.20.2', npm: '9.8.1' }
npm WARN EBADENGINE }
```

Fixes #693

package.json

@@ -6,6 +6,9 @@
   "repository": "git@github.com:cockpit/starter-kit.git",
   "author": "",
   "license": "LGPL-2.1",
+  "engines": {
+    "node": ">= 16"
+  },
   "scripts": {
     "watch": "ESBUILD_WATCH='true' ./build.js",
     "build": "./build.js",