Export `nitroApp` instance

Describe the feature

Context: #3578 (@markthree)

I am currently developing a zero-downtime API service based on H3 and several Nitro applications. I've found that exporting NitroApps might be more user-friendly, as users may need to access their internal options or perform certain actions (such as invoking shutdown hooks).

Example

// plugins/db.ts
// ignore some codes
export default defineNitroPlugin((nitro) => {
    nitro.hooks.hook("close", async () => {
        const db = await useDB();
        db.close();
   });
})

// user case
import("./.output/server/index.mjs").then(({ nitroApp }) => {
   // ignore some codes
   nitroApp.hooks.callHook("close");
})

Additional information

• Would you be willing to help implement this feature?

Start at the generated server entry point .output/server/index.mjs and trace how the Nitro application instance is created and exported. Verify that importing the entry point exposes nitroApp and that its hooks can be invoked as shown; the feature is complete when this public export works for generated Nitro applications.