[js/webgpu] expose a few properties in WebGPU API (#19857)
### Description
This change exposes a few properties in `ort.env.webgpu` to resolve
feature requirement mentioned in properties in
https://github.com/microsoft/onnxruntime/pull/14579#discussion_r1519612619.
- Add `powerPreference` and `forceFallbackAdapter` in `ort.env.webgpu`,
to allow users to set the value of the properties before the first
inference session is created.
- Add readonly property `adapter` in `ort.env.webgpu` to allow users to
get the adapter instance. Now users can access `ort.env.webgpu.device`
and `ort.env.webgpu.adapter`.
@xenova @beaufortfrancois
