Do not extend GeSHi class with private modifications
Our GeSHi class is intended for other extensions making use of GeSHi.
However other libraries or extensions can bring their own GeSHi to the
classloader. In those cases, we cannot find our private modifications
like $compatibleLexers on this class. Instead use
SyntaxHighlightGeSHiCompat for our privact modifications and keep GeSHi
closer to the original implementation.