Add getInitialQuery to documentsearch interface

[aschlaep]

As suggested on #6081 by @gabegrand, this adds getInitialQuery to the ISearchProvider interface. This allows the SearchInstance to get the initial query (set as any because it could be any type of object in the future when we support other search widgets), populate the search box and start a search. To the user, this means that if you have a word selected and press ctrl + f, a search box will open populated with that word, all of the matches will be highlighted, and the selected match will be in yellow. This logic only activates when the search box is initially created, so it won't ever clobber existing text in the search box. Lastly, because search doesn't support newlines, if a selection contains newlines at the time of search box creation, the box will not be populated. @jasongrout

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Bellisimo!