switch to a Document trait and introduce Script/Head/Style/Meta components (#2635)

* switch to a Document trait and introduce Script/Head/Style/Meta components

* Fix desktop title

* Insert special elements into the head during the inital SSR render

* Make all head component attributes optional

* hydrate head elements

* improve the server streaming head warning

* Document fullstack head hydration approach

* deduplicate head elements by href

* move Link into head::Link

* document head components

* add meta and title examples

* Fix a few doc examples

* fix formatting

* Add title to playwright tests

* serde is optional on web, but it is enabled by hydrate

* remove leftover console log

packages/fullstack/src/serve_config.rs

@@ -107,13 +107,19 @@ fn load_index_html(contents: String, root_id: &'static str) -> IndexHtml {
         post_main.1.to_string(),
     );
 
+    let (head, close_head) = pre_main.split_once("</head>").unwrap_or_else(|| {
+        panic!("Failed to find closing </head> tag after id=\"{root_id}\" in index.html.")
+    });
+    let (head, close_head) = (head.to_string(), "</head>".to_string() + close_head);
+
     let (post_main, after_closing_body_tag) =
         post_main.split_once("</body>").unwrap_or_else(|| {
             panic!("Failed to find closing </body> tag after id=\"{root_id}\" in index.html.")
         });
 
     IndexHtml {
-        pre_main,
+        head,
+        close_head,
         post_main: post_main.to_string(),
         after_closing_body_tag: "</body>".to_string() + after_closing_body_tag,
     }
@@ -121,7 +127,8 @@ fn load_index_html(contents: String, root_id: &'static str) -> IndexHtml {
 
 #[derive(Clone)]
 pub(crate) struct IndexHtml {
-    pub(crate) pre_main: String,
+    pub(crate) head: String,
+    pub(crate) close_head: String,
     pub(crate) post_main: String,
     pub(crate) after_closing_body_tag: String,
 }