Default shader target selection in the UI is SPIRV at first load, but a WGSL shader is running

[swoods-nv]

When you first load the playground, the default shader demo is compiled and running in WebGPU, and the target code is displayed beneath the visualization window (in WGSL). It is potentially confusing that the compilation target selector defaults to SPIRV. We should have WGSL be the default target if the default demo is one that will run in the visualizer (and therefore be using WGSL).