Add a #[wasm_bindgen(constructor)] attribute

This can be used for specifying that a static function should actually delegate
to `new`

crates/wasm-bindgen-shared/src/lib.rs

@@ -33,6 +33,7 @@ pub struct ImportStruct {
 #[derive(Serialize, Deserialize)]
 pub struct ImportStructFunction {
     pub method: bool,
+    pub js_new: bool,
     pub function: Function,
 }