Respect package.json's engines.node field when used as a node-version-file (#485)

* Allow reading 'package.json' as node-version-file

* Run 'npm run build'

* Read package.json contents directly during tests

- this eliminates OS-specific line-ending issues

* Run project Prettier 💅

__tests__/data/package.json

@@ -0,0 +1,5 @@
+{
+  "engines": {
+    "node": ">=14.0.0"
+  }
+}