restructure repository as an Octave pkg

Restructure this project as an Octave pkg.

• Move m-files into inst
• Move code into src
• Create a new makefile to build src without autotools
• Ensure src can be built directly from Git, no bootstrap necessary
• Ensure src can still be built out-of-tree
• Ensure src can still be built against different Python runtimes
• Add DESCRIPTION and INDEX package metadata files
• Test that pkg install https://gitlab.com/mtmiller/octave-pythonic/-/archive/master/octave-pythonic-master.tar.gz works