Update server to latest code from repo

[slevithan]

#4 was merged a long time ago, but the changes are still not live at https://www.regexplanet.com/advanced/xregexp/index.html.

This can be tested e.g. by trying the regex (?<1one>.) which should throw an error in XRegExp v5.0.0+ because it has a number as the first character of a capture name.

Updating the server to use the already-merged code would offer significant improvements for XRegExp testing.