Disambiguate quickSuggestion from editor.action.triggerSuggest

From a CompletionItemProvider it would be useful to be able to disambiguate completions triggered by quickSuggest and manual invocation of the command (via ctrl-shift). This could be used if specific completions are expensive or particularly low confidence. Currently quickSuggest suggestions and ctrl-space suggestions both come through with CompletionContext.triggerKind = 0 and triggerCharacter = undefined.