I ran into this while working on keybindings for the pending-keystrokes
indicator. Keystroke interceptors never received standalone modifiers.

GPUI recognizes a standalone modifier on release through a
`ModifiersChangedEvent`. But interception only handled `KeyDownEvent`s,
so the stroke went straight to keymap dispatch. This change passes the
recognized keystroke to interceptors instead of checking the raw event
type.

1. Recording standalone modifiers

With this binding in `keymap.json`, start recording a binding, then
press and release Shift:

```json
[
  {
    "bindings": {
      "shift": "command_palette::Toggle"
    }
  }
]
```

- Before, the command palette opens and Shift is not recorded.
- After, the recorder captures `shift` without opening the command
palette.

2. Searching for standalone modifiers

With these bindings, enable keystroke search, turn exact matching off,
then press and release Control:

```json
[
  {
    "context": "Editor",
    "bindings": {
      "ctrl": "command_palette::Toggle",
      "ctrl-k": "command_palette::Toggle"
    }
  }
]
```

- Before, holding Control finds Ctrl-K but not Control alone. Releasing
Control clears the modifier query.
- After, releasing Control records it as a standalone key, and non-exact
search finds both Ctrl-K and Control alone.

Release Notes:

- Fixed recording and searching for standalone modifier-key bindings in
the keymap editor.
- [GPUI] Fixed keystroke interceptors to receive standalone modifier
keystrokes.