Resolves #1107, Resolves #1111, Resolves #1110

Currently, there are several ways to register a h3 sub-app, as a middleware, route handler or fetch handler. While they work, all are opaque as we lose information about sub-app other than a Request=>Response interface. Additionally, having multiple nested routers adds performance overhead, and we cannot access the route meta of nested apps.

This PR solves all issues by natively supporting mounting an H3 sub-app using app.mount(base, app):

• Nested middleware will be auto-grouped with the base prefix to main app.
• Nested routes will be prefixed and added to the main top-level router.

import { H3, serve, redirect } from "h3";

const nestedApp = new H3().get("/test", () => "/test (sub app)");

const app = new H3()
  .get("/", (event) => redirect(event, "/api/test"))
  .mount("/api", nestedApp);

With this, we encourage .mount over other methods (#1128) and simple routes info is accessable from app._routes (#1127) including new meta (#1118)