enable WebGPU EP in WebAssembly build (#23913)
### Description
This PR is the first step for migrating the webgpu backend of
onnxruntime-web from JSEP based to WebGPU EP based.
In this change, we enable building WebGPU EP in a wasm build (ie.
`--build_wasm` `--use_webgpu` `--use_jsep`). However, the old build
flags should still keep previous behavior.
