Do not use wxRegEx in wxCmpNaturalGeneric()

Using wxRegEx in wxCmpNaturalGeneric() introduced a dependency of the
base library on the regex library.

Replace wxRegEx with character classification functions wxIsspace(),
wxIspunct(), and wxIsdigit() to remove this rather unnecessary
dependency.

Closes https://github.com/wxWidgets/wxWidgets/pull/2014

interface/wx/arrstr.h

@@ -486,9 +486,6 @@ int wxCmpNatural(const wxString& s1, const wxString& s2);
 /**
     This is wxWidgets' own implementation of the natural sort comparison function.
 
-    Requires wxRegEx, if it is unavailable numbers within strings are not
-    recognised and only case-insensitive collation is performed.
-
     @see wxCmpNatural()
 
     @since 3.1.4