url: forbid certain confusable changes from being introduced by toASCII

The legacy url.parse() function attempts to convert Unicode domains
(IDNs) into their ASCII/Punycode form through the use of the toASCII
function. However, toASCII can introduce or remove various characters
that at best invalidate the parsed URL, and at worst cause hostname
spoofing:

  url.parse('http://bad.c℀.good.com/').href === 'http://bad.ca/c.good.com/'
  (from [1])
  url.parse('http://\u00AD/bad.com').href === 'http:///bad.com/'

While changes to the legacy URL parser are discouraged in general, the
security implications here outweigh the desire for strict compatibility.
This is since this commit only changes behavior when non-ASCII
characters appear in the hostname, an unusual situation for most use
cases. Additionally, despite the availability of the WHATWG URL API,
url.parse remain widely deployed in the Node.js ecosystem, as
exemplified by the recent un-deprecation of the legacy API.

This change is similar in spirit to CPython 3.8's change [2] fixing
bpo-36216 [3] aka CVE-2019-9636, which also occurred despite potential
compatibility concerns.

[1]: https://hackerone.com/reports/678487
[2]: https://github.com/python/cpython/commit/16e6f7dee7f02bb81aa6b385b982dcdda5b99286
[3]: https://bugs.python.org/issue36216

PR-URL: https://github.com/nodejs/node/pull/38631


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>