web-sys: Add support for Global-scope methods

This commit adds further support for the `Global` attribute to not only emit
structural accessors but also emit functions that don't take `&self`. All
methods on a `[Global]` interface will not require `&self` and will call
functions and/or access properties on the global scope.

This should enable things like:

    Window::location() // returns `Location`
    Window::fetch(...) // invokes the `fetch` function

Closes #659

crates/backend/src/codegen.rs

@@ -784,6 +784,9 @@ impl TryToTokens for ast::ImportFunction {
                 }
                 class_ty = Some(ty);
             }
+            ast::ImportFunctionKind::ScopedMethod { ref ty, .. } => {
+                class_ty = Some(ty);
+            }
             ast::ImportFunctionKind::Normal => {}
         }
         let vis = &self.function.rust_vis;