Allow emoji characters as well-formed XML

[hata6502]

Resolved jsdom/jsdom#3461 .

https://github.com/jsdom/w3c-xmlserializer/blob/master/lib/serialize.js#L151
w3c-xmlserializer checks the given XML node is well-formed.
Emoji characters (such as 🔍) are not matched to XML_CHAR regular expression.
But, Emoji characters are allowed in well-formed XML according to the W3C specification.

https://github.com/jsdom/w3c-xmlserializer/blob/master/lib/serialize.js#L8
XML_CHAR is defined as /^(\x09|\x0A|\x0D|[\x20-\uD7FF]|[\uE000-\uFFFD]|(?:[\uD800-\uDBFF][\uDC00-\uDFFF]))*$/u
Because this regular expression uses u option, surrogate pair (such as emoji) is not matched.

> (/^(\x09|\x0A|\x0D|[\x20-\uD7FF]|[\uE000-\uFFFD]|(?:[\uD800-\uDBFF][\uDC00-\uDFFF]))*$/u).test("🔍")
   false

Surrogate pair is matched by adding [\u{10000}-\u{10FFFF}] range.

> (/^(\x09|\x0A|\x0D|[\x20-\uD7FF]|[\uE000-\uFFFD]|[\u{10000}-\u{10FFFF}])*$/u).test("🔍")
   true

[domenic]

Thanks for working on this! Can you add a test to https://github.com/jsdom/w3c-xmlserializer/blob/master/test/test.js that fails before this change, and passes afterward?

[hata6502]

Thank you for reviewing!
The test case is added. f37176a
It is passed on this pull request, and failed by reverting 8ef9d81 .