Remove std feature gate for JsError

I'm frankly not sure how this ever worked since wasm-pack seemingly errors on custom error types, but I've not looked too deeply.

Anyways core::error::Error exist so this feature gate should disappear: https://github.com/wasm-bindgen/wasm-bindgen/blob/main/src/lib.rs#L1847

impl<E: core::error::Error> From<E> for JsError {
    fn from(error: E) -> Self {
        use std::string::ToString;
        JsError::new(&error.to_string())
    }
}

It'll need alloc of course, but I guess wasm-bidgen does too.