Add #[wasm_bindgen(assert_no_shim)] on imported functions for testing

This should not be used outside of wasm-bindgen's test suite.

crates/backend/src/ast.rs

@@ -122,6 +122,7 @@ pub struct ImportFunction {
     pub catch: bool,
     pub variadic: bool,
     pub structural: bool,
+    pub assert_no_shim: bool,
     pub kind: ImportFunctionKind,
     pub shim: Ident,
     pub doc_comment: Option<String>,

crates/backend/src/encode.rs

@@ -272,6 +272,7 @@ fn shared_import_function<'a>(
         shim: intern.intern(&i.shim),
         catch: i.catch,
         method,
+        assert_no_shim: i.assert_no_shim,
         structural: i.structural,
         function: shared_function(&i.function, intern),
         variadic: i.variadic,