Implement JsCast for all imported types

This commit implements the `JsCast` trait automatically for all imported types
in `#[wasm_bindgen] extern { ... }` blocks. The main change here was to generate
an `instanceof` shim for all imported types in case it's needed.

All imported types now also implement `AsRef<JsValue>` and `AsMut<JsValue>`

crates/macro-support/src/parser.rs

@@ -524,11 +524,13 @@ impl ConvertToAst<()> for syn::ForeignItemType {
     type Target = ast::ImportKind;
 
     fn convert(self, (): ()) -> Result<Self::Target, Diagnostic> {
+        let shim = format!("__wbg_instanceof_{}_{}", self.ident, ShortHash(&self.ident));
         Ok(ast::ImportKind::Type(ast::ImportType {
             vis: self.vis,
-            name: self.ident,
             attrs: self.attrs,
             doc_comment: None,
+            instanceof_shim: shim,
+            name: self.ident,
         }))
     }
 }