ggml-webgpu: Improve performance of mat-vec and mat-mat for MUL_MAT_ID (#22464)
* Add mat-vec fast path of MUL_MAT_ID.
* Add shared accumulation vec logic and the other types supports.
* Add i-quant mat-mat for MUL_MAT_ID and fix some parts
* Remove n_experts from shader_lib_context.