ggml-webgpu: support for SSM_SCAN and disable set_rows error checking

[reeselevine]

Overview

• Adds support for SSM_SCAN, so that models like Granite can run fully-offloaded to GPU in WebGPU. Uses a subgroup reduction if subgroups supported, otherwise falls back to shared memory reduction.
• Disables the error checking for i64 set rows indexes, because practically the WebGPU backend isn't being run on models big enough for it to matter and I think they would break anyways due to WebGPU's lack of 64-bit types. This also allows graph_compute to be fully non-blocking, CPU/GPU synchronization now only needs to happen in get_tensor.
• Also updated ops support for WebGPU since it hadn't been updated in a bit.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: yes, to investigate ssm_scan kernels and prototype the WebGPU version.