Cleanup Promise in FindUtils

[JeffryBooher]

FindUtils.performReplacements has a blocking promise for the result but needs to wait on CommandManager.execute(FILE_OPEN, ...) rather than use the blocking synchronous private method MainViewManger._edit to open the file in the editor.

[pthiess]

@JeffryBooher How does this surface to the user? Assuming it has important impact I'd say this is a medium priority.

[ingorichter]

@JeffryBooher do you have the time to finish this for 1.1? If so, please add the milestone back.