(bug 45669) Fix highly recursive number highlighting regex

The regular expression used for number highlighting in GeSHi is highly
recursive, and easily overflows the low PCRE recursion limit on WMF
sites (and, on sites where the recursion limit isn't low, it can easily
crash PHP).

Fortunately, it's easy to fix for the common case.

This is also reported upstream at
https://sourceforge.net/p/geshi/bugs/223/

Bug: 45669
Bug: 36839
Change-Id: I27203c767d1d3f2f0999b1b1d8a06e8cf68c19ed