fix: Setting world on FlameGame camera setter

[kurtome]

Description

Currently setting a new FlameGame.camera can cause camera.world to no long match game.world, which I think it's safe to assume is never desirable.

This PR sets the camera.world to the game's world when setting a new Camera, this matches what happens when a new World is set on the game (world.camera is set automatically).

Checklist

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

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

[ufrshubham]

What if the new camera already points to another world?

[kurtome]

What if the new camera already points to another world?

I'm open to changing this to only set _camera.world if it's currently null

[spydon]

@kurtome can you also add a few words about it in the dartdocs? And possibly a tiny test (optional)?

[kurtome]

@spydon I took a crack at the tests, let me know if they are what you had in mind

[spydon]

Good stuff! Thanks for adding the tests too