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/src/errors.rs

@@ -0,0 +1,29 @@
+//! Contains a hydration compatible error boundary context.
+
+use crate::serialize_context;
+use dioxus_core::{spawn_isomorphic, CapturedError, ErrorContext, ReactiveContext};
+use futures_util::StreamExt;
+
+/// Initializes an error boundary context that is compatible with hydration.
+pub fn init_error_boundary() -> ErrorContext {
+    let initial_error = serialize_context().create_entry::<Option<CapturedError>>();
+    let (rx_context, mut rx) = ReactiveContext::new();
+    let errors = ErrorContext::new(None);
+    if let Ok(Some(err)) = initial_error.get() {
+        errors.insert_error(err);
+    }
+    rx_context.run_in(|| {
+        errors.error();
+    });
+    spawn_isomorphic({
+        let errors = errors.clone();
+        async move {
+            if rx.next().await.is_some() {
+                rx_context.run_in(|| {
+                    initial_error.insert(&errors.error(), std::panic::Location::caller())
+                });
+            }
+        }
+    });
+    errors
+}