Expand MergeMeshError to include IncompatiblePrimitiveTopology variant

[Pnoenix]

Objective

Fix #18546 by adding a variant to MergeMeshError, for incompatible primitive topologies.

Solution

Made MergeMeshError into an enum with two variants; IncompatibleVertexAttributes and IncompatiblePrimitiveTopology.
Added an if statement in Mesh::merge to check if the primitive_topology field of self matches other.
Also renamed MergeMeshError to MeshMergeError to align with the two other MeshSomethingError's.

Testing

Didn't do any.

[github-actions]

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[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[Pnoenix]

@alice-i-cecile I added a migration guide file, does it look fine?

[NthTensor]

Could you use a shorter snake case name for the migration guide file? Like merge_mesh_errors.md? @alice-i-cecile do you have preferences about the format?

[alice-i-cecile]

Agreed; short snake_case name that mentions the type that was changed please.