onnxruntime

[js/web] allow ShaderHelper to use internal (non-I/O) variables (#18525)

### Description
This PR includes a change that inspired from #18452 to resolve a
requirement: a shader may depend on an instance of `IndicesHelper` to
generate WGSL code snippet, but the IndicesHelper instance is not
necessarily an input/output of the program. So the existing
`declareVariables()` function does not work with this scenario.

In order to support this requirement, I added this "use" function to
`interface ShaderHelper`, which takes a helper-like object as parameter.
The hidden implementation `ShaderHelperImpl` class will iterate the
helpers and call `impl()` for each.

@axinging @qjia7