The Spark verifier of the custom properties of the overlays

It runs after the transformers, one time for each deck module at compile time. It calls Expresso.Overlay.Properties.slide/2 for each slide with the names of the theme and of the CSS of the deck, and it gives each result to the compiler as a warning.

A warning is not an error. The deck compiles, and the renderer writes the property. Elixir reports the warning of a verifier as a compile warning, and mix compile --warnings-as-errors then fails for a deck module in lib/. A test collects the warning with Spark.Test.dsl_warnings/1.