Add .into() to the generated code for async fns

[MOZGIII]

Without this I am getting an error for declarations like these:

    #[wasm_bindgen(method, getter)]
    pub async fn closed(this: &WebTransport) -> WebTransportCloseInfo;

    #[wasm_bindgen(method, getter, catch)]
    pub async fn closed(this: &WebTransport) -> Result<WebTransportCloseInfo, JsValue>;

I have tried to also create a test for this to catch regressions, but was unable to figure out where to put it.

Test

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen]
    #[derive(Debug, Clone)]
    pub type WebTransportCloseInfo;
}

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen]
    #[derive(Debug, Clone)]
    pub type WebTransport;

    #[wasm_bindgen(method, getter, catch)]
    pub async fn closed(this: &WebTransport) -> Result<WebTransportCloseInfo, JsValue>;
}