macOS: fix incorrect delete symbol mapping (#12011)

`GHOSTTY_KEY_DELETE` should be mapped to `KeyEquivalent.deleteForward`.
This fixes the correct symbol showing in the menu. Previously, both
`GHOSTTY_KEY_DELETE` and `GHOSTTY_KEY_BACKSPACE` were showing `⌫`, but
`GHOSTTY_KEY_DELETE` only worked for `fn+delete`.

Add the following keybind and observe the symbol in the menu:
```
keybind=delete=new_tab
```

<img width="535" height="318" alt="image"
src="https://github.com/user-attachments/assets/67ed7b5d-f848-42ee-a382-fe364d86cb2c"
/>