Add a structural bindgen attribute

This attribute indicates that methods are to be accessed in a structural method
rather than through their class. This should allow direct access to properties
embedded on objects rather than forcing all objects to have a class/prototype.

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

@@ -38,6 +38,7 @@ pub struct ImportFunction {
     pub catch: bool,
     pub method: bool,
     pub js_new: bool,
+    pub structural: bool,
     pub getter: Option<String>,
     pub setter: Option<String>,
     pub class: Option<String>,