Add Sprite3D and AnimatedSprite3D demo

[yuantianle]

Finish demoing the project and implementing Sprite3D and AnimatedSprite3D. Also, provide game visual effects using shaders.

TODO: The link to the demo on the asset library in the README needs to be added.

Related to #1173

[yuantianle]

@Calinou do you have any good ways to eliminate the format issue shown in https://github.com/godotengine/godot-demo-projects/actions/runs/14484485384/job/40627594851?pr=1191? Thanks!

[Calinou]

@Calinou do you have any good ways to eliminate the format issue shown in godotengine/godot-demo-projects/actions/runs/14484485384/job/40627594851?pr=1191? Thanks!

There's trailing whitespace in the files. You can remove it by saving the impacted files with a text editor that has its "Trim Trailing Whitespace on Save" option enabled.

That said, I can handle this since there are many changes that need to be done before the demo can be merged.

[yuantianle]

Thank you! Please let me know if you will need any help.

[yuantianle]

@Calinou Just following up. Are there anything I can help for this PR? Thanks.

[Calinou]

@Calinou Just following up. Are there anything I can help for this PR? Thanks.

I've applied changes to the demo to polish it up, it should be good to merge now.

Thanks a lot for submitting this demo 🙂

[Calinou]

Thanks! Congratulations for your first merged pull request 🎉

[yuantianle]

Thank you @Calinou !~