Keybinding editor could be clearer

### Windows Terminal version (or Windows build number)

Windows Terminal Preview Version: 1.11.2421.0

### Other Software

_No response_

### Steps to reproduce

- open terminal settings
- in "Actions" add a new command

### Expected Behavior

- new command edit working

### Actual Behavior

new command edit not working

This is an example trying to add the first drop-down option "Break into the debugger". The intended key cord was "alt+shift+b". Notice how "a", "l", "t" are echoed in the entry box, then when trying to add "+" it just show "shift+plus":

https://user-images.githubusercontent.com/3933920/134908242-c3c6dc01-eae0-477c-b28a-4586eda3d9a2.mp4

Start by reproducing the issue in Windows Terminal Preview 1.11.2421.0: open Settings, choose Actions, and add a command using the keybinding editor. Compare the editor's handling of `alt+shift+b` with the linked recording; done means the intended key chord is entered and saved as the command binding.

Written by the indexing model from the issue text.