fix: return async function from registerSW in dev mode

[piotr-cz]

Description

The signature for registerSW function should be same for both production and development:

function registerSW(options: RegisterOptions = {}): (_reloadPage?: boolean) => Promise<void>

Linked Issues

None

Additional Context

When trying to update service worker in development, following code throws an error:

import { registerSW } from 'virtual:pwa-register'

const updateServiceWorker = registerSW()

// ...

updateServiceWorker().then(() => { console.log('Service worker updated') })
// Uncaught TypeError: Cannot read properties of undefined (reading 'then')

Note:
Waiting for updateServiceWorker promise to finish this way is particularly helpful to mitigate #789 when used in a react hook

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[userquin]

thx ❤️