macOS: Text from DefaultKeyBindings.dict inserts intermediate characters as well

[mitchellh]

Discussion: #5840

Docs: https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/EventOverview/TextDefaultsBindings/TextDefaultsBindings.html

Reproduction

1. Put the contents below in ~/Library/KeyBindings/DefaultKeyBinding.dict and restart any running programs you want to use it with.
2. Type "HI!"
3. It should get replaced with the emoji but we see intermediate characters as well.

{
  "H" = {
    "I" = {
      "!" = ("insertText:", "👋");
    };
  };
}

[mitchellh]

I can see why this happens, but I'm not sure on the proper robust fix for it. The issue is that if we don't get any text produced by a keypress, we fallback to using NSEvent.characters. We do this for OTHER reasons, so finding a way to do it when we want and avoid it when we don't is going to be the tricky part here.

[mitchellh]

Marking this for 1.3 because the risk is too great for 1.2 (to regress other input)