`wasm_bindgen` on `async fn` doesn't accept same types that it does on regular `fn`

[RReverser]

Describe the Bug

Currently #[wasm_bindgen] on async fn seems to only accept types that are Into<JsValue>, but many wasm-bindgen types are only IntoWasmAbi, making it hard to convert synchronous code into asynchronous.

Steps to Reproduce

Try to compile code like following:

use wasm_bindgen::Clamped;

#[wasm_bindgen]
pub fn foo() -> Clamped<Vec<u8>> {
  Clamped(vec![])
}

#[wasm_bindgen]
pub async fn foo() -> Clamped<Vec<u8>> {
  Clamped(vec![])
}

Expected Behavior

Both functions compile successfully and generate their bindings.

Actual Behavior

Second function fails to compile due to:

   | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ the trait `From<Clamped<Vec<u8>>>` is not implemented for `JsValue`

[t-moe]

For fellow googlers. This is halfway fixed in #3630