Add "extension"

Author: lizmat

S16-io.pod

@@ -311,6 +311,7 @@ order):
   catpath        create a path from volume/directories/filename
   curdir         the path to the current directory (usually '.')
   devnull        the path to the bit bucket (on Unixy systems '/dev/null')
+  extension      the extension of the path
   is-absolute    whether the path is absolute
   join           create a path from hash with volume/directories/filename
   PATH           %ENV<PATH> interpreted as paths
@@ -357,6 +358,7 @@ The following methods are also provided (in alphabetical order):
   contents       files in path (if dir)
   copy           create a copy of file
   directory      the directory part of the absolute path
+  extension      the extension of the file
   is-absolute    is the (original) path absolute
   is-relative    is the (original) path relative
   lines          contents of file as lines