* ggml-webgpu: add CEIL operation support
Add support for the CEIL unary operation in the WebGPU backend:
- Add CEIL_FUNC shader template in unary_op.wgsl
- Add 4 shader variants (f32, f16, inplace versions)
- Initialize CEIL pipelines in ggml-webgpu.cpp
- Register CEIL in supports_op function
* docs: update WebGPU ops support for CEIL