Upgrade to XRegExp 5.0.1

[slevithan]

This makes several improvements:

• Upgrades XRegExp from v3.2.0 to v5.0.1 (the latest version). This is necessary for the latest improvements to XRegExp regex and replacement text syntax.
• Updates all usage of XRegExp in server.js (v3.2.0 was the last version to support XRegExp.install('natives')).
• Fixes an infinite loop that occurs when using a global regex that is capable of matching zero-length strings (this bug currently also affects regexplanet-js).
• Shows the error message to users when there is a regex syntax error.
• Adds named captures to the results that are shown.

I tried to be very careful with my edits, but note that these changes are untested.

[slevithan]

@fileformat Although this change has been merged for years, it still does not seem to be live on https://www.regexplanet.com/advanced/xregexp/index.html