Support animated GIF output via ImageMagick (fixes #102)

[rainerrodrigues]

Description

This PR adds support for generating animated GIFs from saved frame archives, addressing issue #102.

Changes Made

• Added the convert_frames_to_gif function to src/animations.jl.
• Per the maintainer's suggestion in the original issue, this implementation utilizes ImageMagick's convert command via Julia's native run() setup, avoiding the need to add heavy new package dependencies.
• Mirrored the existing architecture of convert_frames_to_video, securely handling the .tar extraction within a mktempdir() block and calculating the appropriate ImageMagick delay based on the requested framerate.
• Exported the new function in src/MeshCat.jl.
• Added a passing test block in test/video_rendering.jl utilizing the existing frames.tar mock data to verify successful .gif generation.

Fixes #102