Fixes
https://linear.app/zed-industries/issue/Z-2552/pressing-two-keystrokes-in-rapid-succession-ignores-the-latter

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, dispatched as a key
equivalent)
- Pressing a character (with or without `shift` held down, dispatched as
a key 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.

Release Notes:

- Fixed a bug that could cause certain keystrokes performed in rapid
succession to incorrectly get ignored.