codify "platform" into resolution logic, server-fn/fullstack overhaul (#4596)

overhaul dioxus fullstack using a new implementation of server_fn

This PR is a complete rewrite of dioxus fullstack designed to improve compatibility with axum and to fix a number of compilation errors based on feature flag unification plumbing through macros.

examples/08-apis/read_size.rs

@@ -0,0 +1,42 @@
+//! Read the size of elements using the MountedData struct.
+//!
+//! Whenever an Element is finally mounted to the Dom, its data is available to be read.
+//! These fields can typically only be read asynchronously, since various renderers need to release the main thread to
+//! perform layout and painting.
+
+use std::rc::Rc;
+
+use dioxus::{html::geometry::euclid::Rect, prelude::*};
+
+fn main() {
+    dioxus::launch(app);
+}
+
+fn app() -> Element {
+    let mut div_element = use_signal(|| None as Option<Rc<MountedData>>);
+    let mut dimensions = use_signal(Rect::zero);
+
+    let read_dims = move |_| async move {
+        let read = div_element.read();
+        let client_rect = read.as_ref().map(|el| el.get_client_rect());
+
+        if let Some(client_rect) = client_rect {
+            if let Ok(rect) = client_rect.await {
+                dimensions.set(rect);
+            }
+        }
+    };
+
+    rsx! {
+        Stylesheet { href: asset!("/examples/assets/read_size.css") }
+        div {
+            width: "50%",
+            height: "50%",
+            background_color: "red",
+            onmounted: move |cx| div_element.set(Some(cx.data())),
+            "This element is {dimensions():?}"
+        }
+
+        button { onclick: read_dims, "Read dimensions" }
+    }
+}