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/defined.rs

@@ -246,6 +246,7 @@ impl ImportedTypes for ast::ImportFunctionKind {
     {
         match self {
             ast::ImportFunctionKind::Method { ty, .. } => ty.imported_types(f),
+            ast::ImportFunctionKind::ScopedMethod { ty, .. } => ty.imported_types(f),
             ast::ImportFunctionKind::Normal => {}
         }
     }