engines block to specify min requirements

Per [the 5.0.0 release notes](https://github.com/tschaub/mock-fs/releases/tag/v5.0.0), support for Node less than 12 has been dropped. With an `engines` block, `npm` can issue an error to tell folks on Node 10 (or other lower/unsupported version) that the package is no longer compatible. This can save time for things like #334 where it's not obvious why issues are occurring.

Author: tillig

package.json

@@ -48,5 +48,8 @@
     "mocha": "^8.4.0",
     "rimraf": "^3.0.2",
     "semver": "^7.3.5"
+  },
+  "engines": {
+    "node": ">=12.0.0"
   }
 }