Add importing strings as JsString

[daxpedda]

This PR adds the ability to import strings directly as JsString. This can be useful when trying to avoid TextDecoder/Encoder, e.g. when dealing with an audio worklet.

#[wasm_bindgen]
// Currently Rustfmt simply removes the value on this static.
// See <https://github.com/rust-lang/rustfmt/issues/6267>.
#[rustfmt::skip]
extern "C" {
    #[wasm_bindgen(static_string)]
    static STRING: JsString = "foo";
}

I added the static_string attribute item requirement because we might want to support other types in the future and we can't do type checking on the proc-macro level without knowing what the type should be. The alternative would be to do the type checking via wasm-bindgen by using descriptors, which isn't ideal for users.

Fixes #4031.