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/shared/src/lib.rs

@@ -48,7 +48,7 @@ pub struct ImportFunction {
 
 #[derive(Deserialize, Serialize)]
 pub struct MethodData {
-    pub class: String,
+    pub class: Option<String>,
     pub kind: MethodKind,
 }