Move the document trait into a separate crate (#3035)

* add a default head method through eval
* remove the old document trait
* implement document for each platform
* pull out document into a dedicated crate to cut down on shared dependencies
---------

Co-authored-by: Jonathan Kelley <jkelleyrtp@gmail.com>

examples/todomvc.rs

@@ -63,7 +63,7 @@ fn app() -> Element {
     };
 
     rsx! {
-        head::Link { rel: "stylesheet", href: STYLE }
+        document::Link { rel: "stylesheet", href: STYLE }
         section { class: "todoapp",
             TodoHeader { todos }
             section { class: "main",