[js/webnn] update API of session options for WebNN

[fs-eire]

Description

This PR is an API-only change to address the requirements being discussed in #20729.

There are multiple ways that users may create an ORT session by specifying the session options differently.

All the code snippet below will use the variable webnnOptions as this:

const myWebnnSession = await ort.InferenceSession.create('./model.onnx', {
   executionProviders: [
     webnnOptions
   ]
});

The old way (backward-compatibility)

// all-default, name only
const webnnOptions_0 = 'webnn';

// all-default, properties omitted
const webnnOptions_1 = { name: 'webnn' };

// partial
const webnnOptions_2 = {
  name: 'webnn',
  deviceType: 'cpu'
};

// full
const webnnOptions_3 = {
  name: 'webnn',
  deviceType: 'gpu',
  numThreads: 1,
  powerPreference: 'high-performance'
};

The new way (specify with MLContext)

// options to create MLcontext
const options = {
  deviceType: 'gpu',
  powerPreference: 'high-performance'
};

const myMlContext = await navigator.ml.createContext(options);

// options for session options
const webnnOptions = {
  name: 'webnn',
  context: myMlContext,
  ...options
};

This should throw (because no deviceType is specified):

const myMlContext = await navigator.ml.createContext({ ... });
const webnnOptions = {
  name: 'webnn',
  context: myMlContext
};

Interop with WebGPU

// get WebGPU device
const adaptor = await navigator.gpu.requestAdapter({ ... });
const device = await adaptor.requestDevice({ ... });

// set WebGPU adaptor and device
ort.env.webgpu.adaptor = adaptor;
ort.env.webgpu.device = device;

const myMlContext = await navigator.ml.createContext(device);
const webnnOptions = {
  name: 'webnn',
  context: myMlContext,
  gpuDevice: device
};

This should throw (because cannot specify both gpu device and MLContext option at the same time):

const webnnOptions = {
  name: 'webnn',
  context: myMlContext,
  gpuDevice: device,
  deviceType: 'gpu'
};

[fs-eire]

@Honry @huningxin @egalli Please take a look and let me know if you have any concerns.

[huningxin]

LGTM with questions, thanks much!

[Honry]

Cool, this makes things very clear and fulfills all the requirements.

Thanks much @fs-eire!

[egalli]

LGTM, thanks for making all the API changes!