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/webidl-tests/global.js

@@ -0,0 +1,4 @@
+global.global_no_args = () => 3;
+global.global_with_args = (a, b) => a + b;
+global.global_attribute = 'x';
+