url: enforce valid UTF-8 in WHATWG parser

This commit implements the Web IDL USVString conversion, which mandates
all unpaired Unicode surrogates be turned into U+FFFD REPLACEMENT
CHARACTER. It also disallows Symbols to be used as USVString per spec.

Certain functions call into C++ methods in the binding that use the
Utf8Value class to access string arguments. Utf8Value already does the
normalization using V8's String::Write, so in those cases, instead of
doing the full USVString normalization, only a symbol check is done
(`'' + val`, which uses ES's ToString, versus `String()` which has
special provisions for symbols).

PR-URL: https://github.com/nodejs/node/pull/11436


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>