Add stdweb feature like wasm_bindgen feature
gilrs-core currently has a mandatory dependency to stdweb, even when the wasm_bindgen feature is activated.
Generally, disabling functionality (stdweb support in this case) by activating a feature (wasm_bindgen) is discouraged (features should be additive). That's why optional dependencies can't be activated when a feature is deactivated in Cargo.
Other crates (like winit) solved this issue with two different features for stdweb and wasm_bindgen/web-sys.
I would like to create a merge request if this is acceptable.