Add support for Pixi and some more pandocomatic V2.0 tweaks

1. https://pixi.sh is a new package manager (combining both homebrew and pyenv / rbenv functionality) that is cross-platform, fast and with many available packages as it uses conda forge. It's a bit less flexible than rbenv for multiple versions of the same tool, but if all you want is the latest ruby and gem installed globally then pixi works well.
2. Pandocomatic V2.x added a --log option. If scrivomatic is passed the -v verbose flag then this is enabled and a pandocomatic.log file is output in the working directory. Note we also pass --enable pandoc-verbose since V1.0.38