Make pymupdf the default tool for generating svg and png

[oscarlevin]

from latex-image. The original method is still available, behind a pymupdf switch that now defaults to true.

[rbeezer]

This is looking good, though I have not yet taken it for a drive. Thanks for preserving the old behavior. I think it needs two related things, and one follow-on:

• Documentation should stop talking about installing/using pdf2svg and PyMuPDF should be added as a dependency for the pretext/pretext script running in a Python virtual environment.
• Lots of project files and pretext.cfg files in the repository pointing at pdf2svg. These need to change so the website update script won't be impeded.
• I'm only likely to update SVG images for the sample article as a test. A subsequent PR could update others in the repository.

[oscarlevin]

Added two commits to address your first two bullets. Also added a new issue to eventually completely remove convert (#2188 )

[rbeezer]

Thanks, Oscar! Sorry for the long delay. I thought this was a bit more involved than it really is/was. Merged as-is, but for one whitespace character.

I'll suggest @Alex-Jordan remove the final dependency on convert.

Great to get rid of all those instructions in the Guide about one-off software installations.