Replace Node's deprecated punycode

[bcomnes]

Importing markdown-it into Node.js triggers a warning now:

node:11893) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead.

Looks like anything relying on Node.js's punycode implementation should switch to a userland implementation.

markdown-it/lib/index.js

Line 14 in e476f78

var punycode = require('punycode');

Let me know if you want a PR.

[philoserf]

see also: DavidAnson/markdownlint-cli2#234 and igorshubovych/markdownlint-cli#437 and DavidAnson/markdownlint#1032

or more broadly: https://github.com/search?q=DEP0040++&type=issues&state=open

[philoserf]

Thank you. Most downstream users will not get an update until we tag/release.

[puzrin]

#863 (comment) in progress.