PL/I: Also recognise "pl1" extension and MIME type
The file extension ".pl1" is used for Multics source (along with ".cds" and ".rd" which are also PL/I source, but are rarer and may clash with other types so are not added).
The original MIME type "text/x-pli" is not defined in shared-mime-info and does not seem to be used anywhere online. The type "text/x-pl1src" has been defined and used for Multics files by https://github.com/martenjj/multics-mimetypes.
Adding these definitions allows Multics PL/I source files to be recognised and highlighted.