Include dependency if feature isn't specified

I don't want to treat wasm32 as the browser, so I had to add a feature that indicates that the build is occuring for the browser platform, which will be added via my CLI to cargo build.

# multi-threaded target only dependencies
rialight_multi_threaded_export = [
    "tokio",
]

# browser export only dependencies
rialight_browser_export = [
    "stdweb",
    "web-sys",
    "wasm-bindgen-futures",
]

The problem is, there's no way to do this instead of using an unnecessary rialight_multi_threaded_export feature:

['cfg(not(feature(rialight_browser_export)))'.dependencies]
tokio = "version"