[w3c/manifest] Provide a way to not open a link in the PWA, in the moment (#989)

Many web apps have a login. This login page is often opened in a new window, in order not to interrupt the already running web application. However when a web app is installed as PWA, `window.open()` will always open the link in the PWA.

The usual flow in single-page applications is:
- Subscribe to the `window.onmessage` event
- Use `window.open()` to navigate to the URL hosting the login page (for eg. Microsoft OAuth)
- Host an empty html page with only javascript, using the messaging api to notify the `opener`.
- Let the OAuth flow use this page as redirect_url.

But since `window.open()` always opens the PWA, the external-login flow is interrupted.
The bottom line is: a developer MUST BE allowed to have window.open open the url in a new window and prevent it from opening the PWA.

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https://github.com/w3c/manifest/issues/989