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 was reported upstream last time it occurred at
https://sourceforge.net/p/geshi/bugs/223/

Bug: T76352
Change-Id: I9e1f002838ffa546c12baf88517069c78a5e7b9d
(cherry picked from commit f21f0484430ca04164d63a7616972ac8f03b4179)