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/window_event.rs

@@ -26,7 +26,7 @@ fn main() {
 
 fn app() -> Element {
     rsx!(
-        head::Link { href: "https://unpkg.com/tailwindcss@^2/dist/tailwind.min.css", rel: "stylesheet" }
+        document::Link { href: "https://unpkg.com/tailwindcss@^2/dist/tailwind.min.css", rel: "stylesheet" }
         Header {}
         div { class: "container mx-auto",
             div { class: "grid grid-cols-5",