feat: expose layouts and pages on App (#3086)

This PR extracts the functionality from the `fs_routes` plugin and
exposes it onto `App`. These are the new APIs:

- `app.layout(pattern, route)`
- `app.onError(pattern, route)`
- `app.notFound(pattern, route)`
- `app.route(pattern, route)`

This also removes the need for `.use(staticFiles())`. This line can be
safely removed.

Fixes #3040
Fixes #3033
Fixes #2591
Fixes #950
Fixes #1918

Author: marvinhagemeister

init/src/init.ts

@@ -360,13 +360,11 @@ ${GRADIENT_CSS}`;
     // Skip this and be silent if there is a network issue.
   }
 
-  const MAIN_TS = `import { App, fsRoutes, staticFiles } from "fresh";
+  const MAIN_TS = `import { App, fsRoutes } from "fresh";
 import { define, type State } from "./utils.ts";
 
 export const app = new App<State>();
 
-app.use(staticFiles());
-
 // this is the same as the /api/:name route defined via a file. feel free to delete this!
 app.get("/api2/:name", (ctx) => {
   const name = ctx.params.name;