Fix ctrl-d/u issues with scroll_beyond_last_line off (#15395)

Closes #15356

Release Notes:

- vim: Fixed issues with `ctrl-d`/`ctrl-u` when
`scroll_beyond_last_line` is set to `off`
([#15356](https://github.com/zed-industries/zed/issues/15356)).


https://github.com/user-attachments/assets/d3166393-4a4e-4195-9db6-3ff1d4aeec78

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Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

docs/src/vim.md

@@ -258,6 +258,8 @@ There are also a few Zed settings that you may also enjoy if you use vim mode:
   "relative_line_numbers": true,
   // hide the scroll bar
   "scrollbar": { "show": "never" },
+  // prevent the buffer from scrolling beyond the last line
+  "scroll_beyond_last_line": "off",
   // allow cursor to reach edges of screen
   "vertical_scroll_margin": 0,
   "gutter": {