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

example-projects/fullstack-hackernews/src/main.rs

@@ -36,7 +36,7 @@ pub fn App() -> Element {
 #[component]
 fn Homepage(story: ReadOnlySignal<PreviewState>) -> Element {
     rsx! {
-        head::Link { rel: "stylesheet", href: asset!("./assets/hackernews.css") }
+        document::Link { rel: "stylesheet", href: asset!("./assets/hackernews.css") }
         div { display: "flex", flex_direction: "row", width: "100%",
             div {
                 width: "50%",