[WebNN EP] Support Trilu op

[peishenyan]

Adds support for Trilu via WebNN Triangular op

[Honry]

Please add this new op to op_map in helper.h, update the unit tests under /js/web/test/suite-test-list.jsonc, also the js/web/docs/webnn-operators.md

[peishenyan]

Thanks for the comments. I have added the op map in helper.h, updated the webnn-operators.md, and addressed the comments.

PTAL, @Honry @fdwr @guschmue Thanks!

[guschmue]

/azp run ONNX Runtime Web CI Pipeline,Windows GPU CI Pipeline, Linux Android Emulator QNN CI Pipeline

[guschmue]

/azp run Linux CPU CI Pipeline,Linux CPU Minimal Build E2E CI Pipeline,Linux GPU CI Pipeline,Linux GPU TensorRT CI Pipeline,Linux OpenVINO CI Pipeline,Linux QNN CI Pipeline,MacOS CI Pipeline,Windows ARM64 QNN CI Pipeline,Windows CPU CI Pipeline

[guschmue]

/azp run Windows GPU TensorRT CI Pipeline,onnxruntime-binary-size-checks-ci-pipeline,orttraining-linux-ci-pipeline,orttraining-linux-gpu-ci-pipeline,orttraining-ortmodule-distributed,Windows x64 QNN CI Pipeline,Big Models

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