[Virtual Keyboard API] Return promise for Show/Hide calls?

[travisleithead]

Porting over from MSEdgeExplainers repo MicrosoftEdge/MSEdgeExplainers#415

@snianu filed:

In the TPAC VK API session, there were comments about returning a promise when show/hide is called so the web author knows if the call was successful or not. There are also platform specific restrictions that prevent the VK to come up even after app calls show explicitly to raise the VK. Not sure if we can deterministically determine if the show/hide call was successful, but should we return a promise indicating that the call was successful? Also if there was an error (e.g. no user activation or no editable element in focus?) then what is the author supposed to do in that case?

Tagging few folks to comment on this issue. @whsieh @rniwa @othermaciej @mustaqahmed