[js/web] add target ort.webgpu.min.js (#15780)
### Description
add target ort.webgpu.min.js
WebGPU is experimental feature, so I don't want to put webgpu into the
ort.min.js file. This change adds 2 ways for users to access ort-web
with webgpu:
- using script tag: by URL
`https://cdn.jsdelivr.net/npm/onnxruntime-web@1.15.0/dist/ort.webgpu.min.js`
( this URL is not ready yet )
- using `import()`: use `import { Tensor, InferenceSession } from
'onnxruntime-web/webgpu';` - 'onnxruntime-web/webgpu' instead of
'onnxruntime-web'
