Backends: WebGPU: detect WGSL support at runtime instead of excluding WGVK at compile time. (#9387)

Previously WGVK was hard-disabled from WGSL via #if !defined(IMGUI_IMPL_WEBGPU_BACKEND_WGVK),
forcing the SPIRV fallback unconditionally. Now the WGSL path is attempted on all backends
and an empty stage_desc is returned when the module fails to compile letting the existing
SPIRV fallback at the call site kick in.