feat: Make it easier work with the Mesh class [flame_3d]

[luanpotter]

Description

Currently it is quite hard to operate with the Mesh class, because the list of surfaces is kept private.

I understand the concern with immutability, and the fact that the aabb must be recalculated when anything change. Sadly I don't think Dart provides us with patterns to fully automate updating the aabb. We need a way to update the meshes, so this is a compromise:

• add a getter to an unmodifiable list
• if the user updates the surfaces within, they are responsible for calling updateBounds
• add methods to update or remove elements from the list, which automatically call updateBounds
• this allow us to remove the silly "updateMaterial" methods that are used by Defend the Donut

This also adds an add method that takes a Surface object witch is extremely handy (even though it allows for post-modifications).

If we prefer to go a truly fully immutable route, we instead need helper methods to copy and recreate meshes with given set of changes. I am happy to take a stab at that option too.

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?

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