mkdir: return 403 error for mkdir action in the root dir

[XiaochenCui]

fix: #1348

This PR adds check in the "mkdir" logic.
Backend now throws a "403 forbidden" instead of "500 internal server error" when creating dir in the root.

This PR has been tested manually for the following cases:

// return "403 forbiddend" error
puter.fs.mkdir('/foo')

// create successfully
//
// "/bold_toy_921/Videos" is a dir created automatically when running puter locally
puter.fs.mkdir('/bold_toy_921/Videos/foo')

// "readdir" should be able to see "foo" dir now
await puter.fs.readdir('/bold_toy_921/Videos/')

[XiaochenCui]

hey @jelveh
this pr is ready for merge