Previously, we would only track whether the previous key down event
was a key equivalent. However, this could cause issues when pressing
certain keystrokes in rapid succession, e.g.:

- Pressing `shift-right` (to select a character)
- Pressing a character (with or without `shift` held down)

This would cause GPUI to ignore the second event because it was
preceded by a key equivalent event. With this commit, we track the
last key equivalent event, and skip the key down event only if it
matches the last key equivalent event.