* webgpu : add CONV_2D_DW (depthwise conv2d) kernel
Implement GGML_OP_CONV_2D_DW for the WebGPU backend,
ported from the Vulkan backend's conv2d_dw.comp.
Assisted-by: Claude Opus-4.8
* Remove unnecessary comments in webgpu support
* update supported ops tables, triggered by adding webgpu CONV_2D_DW
