fix: Disallow mutatation of `SpriteAnimationGroupComponent.animations`

[spydon]

Description

Many users try to modify the SpriteAnimationGroupComponent.animations map, but that doesn't work since it has to go through the setter, so this PR makes the returned map from the getter unmodifiable.

Checklist

• [x] I have followed the Contributor Guide when preparing my PR.
• [x] I have updated/added tests for ALL new/updated/fixed functionality.
• [x] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [x] I have updated/added relevant examples in examples or docs.

Breaking Change?

• [x] No, this PR is not a breaking change.

Related Issues

[erickzanardo]

Is it the same way in the SpriteGroupComponent?

[spydon]

Is it the same way in the SpriteGroupComponent?

I thought it didn't, but it does!

[spydon]

@erickzanardo this is the follow-up to fix it for SpriteGroupComponent: https://github.com/flame-engine/flame/pull/3185

[erickzanardo]

@erickzanardo this is the follow-up to fix it for SpriteGroupComponent:

https://github.com/flame-engine/flame/pull/3185

Cool, I am both right and late hahahha