[Web] Rename WebNN EP MLOperandDescriptor's "dimensions" key to "shape"

[huningxin]

Describe the issue

Align with spec and Chromium implementation change:

https://chromium-review.googlesource.com/c/chromium/src/+/5502631

webmachinelearning/webnn#676

/cc @Honry @fdwr

To reproduce

See the spec PR

Urgency

No response

ONNX Runtime Installation

Built from Source

ONNX Runtime Version or Commit ID

Latest

Execution Provider

WebNN EP

[fdwr]

For now we can declare MLOperandDescs with both fields (shape is an alias of dimensions), so that we have some transitional overlap across Chromium versions without worrying about exactly matching. e.g.:

const someOperandDesc = {
    dataType: "float32",
    dimensions: listOfDimensions
    shape: listOfDimensions
}