Fix private-use singleton without a subtag being accepted

[spokodev]

A trailing private-use singleton with no subtags is silently accepted and dropped.

import {parse, stringify} from 'bcp-47'

const schema = parse('en-x')
// { language: 'en', privateuse: [], ... } — no warning
stringify(schema) // 'en'  (the `-x` is lost, round-trip broken)

Per RFC 5646, privateuse = "x" 1*("-" (1*8alphanum)), so x must be followed by at least one subtag. The dangling-extension-singleton equivalent (en-i) is already rejected with "Found superfluous content after tag", but the private-use branch is not.

The entry check only verifies the x singleton is present, not that a subtag follows, so the -x is consumed and index advances to the end, leaving no superfluous content to report. Fix: require - + an alphanumeric after x before entering the private-use section, mirroring the extension entry guard. en-x now falls through to the existing superfluous-content check, matching en-i.

Adds a regression test. Valid tags (x-whatever, en-x-a, az-Arab-x-AZE-derbend, …) are unaffected; all existing tests and fixtures pass.