Deprecate relative asset paths, better warnings for asset!() (#3214)

* Deprecate relative asset paths

* fix paths, better warnings in asset parser

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

@@ -36,7 +36,7 @@ pub fn App() -> Element {
 #[component]
 fn Homepage(story: ReadOnlySignal<PreviewState>) -> Element {
     rsx! {
-        document::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%",