Typescript definition not generated for string enums

[Vlad-Shcherbina]

To reproduce

1. Run wasm_bindgen --target web on the wasm produced from the following library:

   use wasm_bindgen::prelude::*;
   
   #[wasm_bindgen]
   pub enum Hand {
       Left = "Left",
       Right = "Right",
   }
   
   #[wasm_bindgen]
   pub fn f(h: Hand) -> Hand { h }    

2. Inspect generated .d.ts

Expected Behavior

export type Hand = 'Left' | 'Right';

export function f(h: Hand): Hand;

Actual Behavior

/**
* @param {any} h 
* @returns {any} 
*/
export function f(h: any): any;

Version info

wasm-bindgen 0.2.62

[alexcrichton]

I unfortunately don't have the time to implement a new feature like this, but a PR would be much appreciated!