Fix :attr:.ManimConfig.format not updating movie file extension

[JasonGrace2282]

Closes #3837

Example of Bug

>>> from manim import config
>>> config.movie_file_extension
'.mp4'
>>> config.format = "webm"
>>> config.movie_file_extension
'.mp4'
>>> config.transparent = config.transparent
>>> config.movie_file_extension
'.webm'

[behackl]

LGTM

[behackl]

should we add a test for this?

[JasonGrace2282]

Good idea, I'm not sure how I forgot that :)

[behackl]

It is somewhat odd how differently we are treating the case of .gif, but that's a different discussion.

[JasonGrace2282]

I think it's just a missed edge case, probably worth fixing (wow we have a lot of problems with ManimConfig xD)