Move `VXYZ` and `EToV` into a `VertexMappedMesh` mesh type

[jlchan]

This removes the VXYZ and EToV fields from MeshData and refactors them into a VertexMappedMesh mesh type. The HybridMesh and NonConformingMesh types have similarly been updated. This avoids having "dummy" fields for cut cell meshes, and should also allow us to dispatch on MeshData{NDIMS, MeshType} and distinguish between, for example, vertex-mapped meshes and curved HOHQmesh.jl meshes.

[codecov[bot]]

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