on linux with the emacs keymap, holding `ctrl-n` at the end of the
buffer opens a new file every press. `ctrl-p` at the top opens the file
finder.

the keymap already tries `"ctrl-n": null`, but `null` only breaks
resolution in a *user* keymap, so from a base keymap
`workspace::NewFile` still resolves. a targeted unbind works regardless
of source:

```json
"ctrl-p": ["zed::Unbind", "file_finder::Toggle"],
"ctrl-n": ["zed::Unbind", "workspace::NewFile"],
```

left the MoveDown/MoveUp propagation alone, it's load bearing (agent
panel uses it for up-to-edit-last-message). tests cover the fix plus the
narrower ctrl-n/ctrl-p bindings still winning, since the unbind only
targets the one action.

ctrl-g/ctrl-x in that block are still null and probably just as broken,
but ctrl-x is a prefix key so i left them.

Closes #45010.

Release Notes:

- Fixed `ctrl-n` opening a new file and `ctrl-p` opening the file finder
at the ends of a buffer when using the Emacs keymap on Linux