waterfallProof – suggested usability improvements

[frankrolf]

• the suggested _content directory is a bit difficult to access once the package has been installed. In my case, in a non-editable installation, it’s at /Users/fg/.pyenv/versions/3.11.11/lib/python3.11/site-packages/drawbot_proofing/_content/waterfall_vertical.txt. It would make more sense to allow specifying external files.
• vertical and horizontal output should be proofing modes, not both output to the same PDF. That PDF displays weirdly (because of the different page formats), and the use case is different.
• in the arguments, the shorthand and long name for the --point-size argument are flipped