[w3c/manifest] Control which parts of the URLs belong to the app and which parts are not (#996)

(I originally posted a related issue at [manifest-incubations](https://github.com/WICG/manifest-incubations/issues/23))

Although there is a scope concept in service worker and manifest file, **for URLs in the scope**, developers need a way to tell the browser which URLs can be regarded as the app and which URLs are not.

Why need to tell browser this information?
<img width="333" alt="install" src="https://user-images.githubusercontent.com/776467/129702587-3f3f6167-6c09-4b56-96d6-99bfadd02a8a.png">
<img width="340" alt="launch" src="https://user-images.githubusercontent.com/776467/129702593-2ff9b252-7e64-481f-8c37-5a765bbb32a9.png">

As shown in the screenshots above, Chrome address bar has install/launch icon for installing and launching the app. But not all URLs in the origin/scope should be regarded as the app. For example, help pages of the app should be opened as web pages in the browser, rather than load them into the app.

A core concept: whether a URL in the scope should be treated as the app. 
- For a web page url, it should not be loaded in the (standalone installed) app, it should be opened in the browser.
- For an app url, it can be loaded in the app.




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