WebGPU: Fix compile errors with newer Dawn versions after WGPUProgrammableStageDescriptor rename

[PhantomCloak]

Dawn renamed WGPUProgrammableStageDescriptor type to WGPUComputeState (webgpu-native/webgpu-headers#413) which breaks the current Dawn backend when building against newer versions. WGPU hasn't renamed it yet, so adding type alias as temporary fix until both backends adopt the same naming.

Compile errors encountered with newer versions of Dawn.

/home/phantom/Developer/Rain2/Rain-EngineUpgrade/vendor/imgui/backends/imgui_impl_wgpu.cpp:266:8: error: unknown type name 'WGPUProgrammableStageDescriptor'
static WGPUProgrammableStageDescriptor ImGui_ImplWGPU_CreateShaderModule(const char* wgsl_source)
       ^
/home/phantom/Developer/Rain2/Rain-EngineUpgrade/vendor/imgui/backends/imgui_impl_wgpu.cpp:283:5: error: unknown type name 'WGPUProgrammableStageDescriptor'
    WGPUProgrammableStageDescriptor stage_desc = {};
    ^
/home/phantom/Developer/Rain2/Rain-EngineUpgrade/vendor/imgui/backends/imgui_impl_wgpu.cpp:668:5: error: unknown type name 'WGPUProgrammableStageDescriptor'
    WGPUProgrammableStageDescriptor vertex_shader_desc = ImGui_ImplWGPU_CreateShaderModule(__shader_vert_wgsl);
    ^
/home/phantom/Developer/Rain2/Rain-EngineUpgrade/vendor/imgui/backends/imgui_impl_wgpu.cpp:690:5: error: unknown type name 'WGPUProgrammableStageDescriptor'
    WGPUProgrammableStageDescriptor pixel_shader_desc = ImGui_ImplWGPU_CreateShaderModule(__shader_frag_wgsl);
    ^
4 errors generated.