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.
* return a 404 when paring the route fails
* add a playwright test for fullstack routing status codes
* Fix new playwright test
* document SSRError
* Make SSRError public and fix clippy
* Move the error display bound to the routable trait
* fix clippy
* return a more generic 404 message
* fix router tests
Jonathan Kelley