Contour plots with Makie.jl and Plots.jl

Sometimes, contour plots are preferred since that's the usual way to represent results in some fields. It would be nice to extend the recently improved Makie.jl support to this case.

CC @MarcoArtiano (who uses contour plots in the context of atmospheric simulations) and @vincmarks (who implemented Makie.jl support in Trixi.jl recently).

No files or tests are named in the issue. Start by reading the recently improved Makie.jl support and the existing Plots.jl plotting entry points, then determine how contour plots should fit both integrations; done means contour plots are supported for the relevant plotting backends and covered by the project’s existing plotting checks.

Written by the indexing model from the issue text.