punycode: update to v1.2.3

punycode@1.2.3 offers improved support for unpaired surrogates.

(punycode@1.2.2 has better export logic which makes it work in various environments. This makes no difference for Node. More unit tests were added, but the core code functionality hasn’t changed. TL;DR Just keeping Node.js’s punycode.js in sync with the main repo.)

I’ve improved the punycode docs too. See the separate pull request for that: #5620.