Support gif animations

[buhtz]

Would be great to have support for animated picture formats like gif (or animated svg, or apng or something else) instead of mp4 files.

[initialcommit-io]

@buhtz Manim does support gif output and I tried briefly to get this working with Git-Sim. However, the process conflicted with the way Git-Sim currently handles video/image output (using respective flags).

So I think it should be possible, but need to figure out the best way to work it into the current Git-Sim output format logic.