[w3c/manifest] Add ability for PWAs to be registered as scheme/protocol handlers (#846)

Native applications often register themselves as protocol handlers to increase discoverability and usage. It would be great to have PWAs do the same.

While websites do currently have this ability via `registerProtocolHandler()` (which for the most part just registers the browser as the handler to the OS), it would be interesting to have PWAs be first-class citizens and be launched when a custom-scheme link is invoked. For instance, on Windows 10, one can choose default apps by protocol, and one would be able to point to an installed PWA to handle that protocol:

![image](https://user-images.githubusercontent.com/1686833/73107241-2ee3c000-3eb2-11ea-81c9-3b001ba3172c.png)

#### Explainer:
https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/master/URLProtocolHandler/explainer.md

#### Related issue:  
It looks like this proposal shares some similarities with #626 (Add a means to associate a PWA with a file extension), and we should probably discuss if they should be unified or kept separate.


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https://github.com/w3c/manifest/issues/846