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>

examples/counters.rs

@@ -16,7 +16,7 @@ fn app() -> Element {
     let sum = use_memo(move || counters.read().iter().copied().sum::<i32>());
 
     rsx! {
-        head::Link { rel: "stylesheet", href: STYLE }
+        document::Link { rel: "stylesheet", href: STYLE }
 
         div { id: "controls",
             button { onclick: move |_| counters.write().push(0), "Add counter" }