property ReactRouterPluginOptions.loadRequestHandler
Since 0.2.0 Returns an {@linkcode SsrRuntime} rather than a bare handler,
Injectable seam for lazy loading the RR runtime. When omitted, the default
performs await import(serverBuildPath) + await import('npm:react-router@8').
Also the supported hook for a development server: return a handler built
over a build thunk (createRequestHandler(() => vite.ssrLoadModule(...)))
to pick up rebuilds without restarting the process.
(serverBuildPath: string,mode: string) => Promise<SsrRuntime>