id is an arbitrary string assigned by the user. id
must be unique per pcb-rnd session per script and may contain alphanumeric characters
and undescrore.
fn is the file name of the script, which is a path. Pcb-rnd does not
do any search, it only opens the path as specified.
lang is the scripting language, as in
fungw plugin name.
When not specified, the code makes a guess (based on the file name).