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/playwright-tests/fullstack-errors.spec.js

@@ -4,6 +4,10 @@ const { test, expect } = require("@playwright/test");
 test("errors", async ({ page }) => {
   await page.goto("http://localhost:3232");
 
+  // give the page time to finish loading resources
+  await page.waitForLoadState("networkidle");
+  await page.waitForTimeout(2000);
+
   // Make sure the error that was thrown on the server is shown in the error boundary on the client
   const errors = page.locator("#error-fallback-button");
   await expect(errors).toContainText("Error fallback button clicked 0 times");