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.

packages/fullstack-core/README.md

@@ -0,0 +1,35 @@
+# Dioxus Fullstack Core
+
+Dioxus-fullstack-core provides types, traits, hooks, contexts for [`dioxus-fullstack`](https://crates.io/crates/dioxus-fullstack). Libraries that need to integrate with dioxus-fullstack should rely on this crate instead of the full-fledged renderer for quicker build times.
+
+## Usage
+
+To start using this crate, you can run the following command:
+
+```bash
+cargo add dioxus-fullstack-hooks
+```
+
+Then you can use hooks like `use_server_future` in your components:
+
+```rust
+use dioxus::prelude::*;
+
+
+fn App() -> Element {
+    let mut article_id = use_signal(|| 0);
+
+    // `use_server_future` will spawn a task that runs on the server and serializes the result to send to the client.
+    // The future will rerun any time the
+    // Since we bubble up the suspense with `?`, the server will wait for the future to resolve before rendering
+    let article = use_server_future(move || fetch_article(article_id()))?;
+
+    rsx! {
+        "{article().unwrap()}"
+    }
+}
+
+async fn fetch_article(id: u32) -> String {
+    format!("Article {}", id)
+}
+```