core directory (which is tiny compared to the plugins)
-- SvenDowideit - 20 Feb 2008
There was a script to create a new plugin at the root of the former twikiplugins I can't find it now.
-- StephaneLenclud - 24 Feb 2008
It's at the root level of the BuildContrib. core subdirectory is)
perl BuildContrib/create_new_extension.pl