ggml : add WebGPU backend

[jakexcosme]

Note: This issue was copied from ggml-org#7773

Original Author: @ggerganov
Original Issue Number: ggml-org#7773
Created: 2024-06-05T14:24:37Z

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I hope that this would be relatively easy to do since AFAIK WebGPU allows us to write kernels in a shader language, so we have experience how to create such backends.

There has been some initial work in ggml-org/ggml#585 - could be useful as a starting point