Fix vim <CTRL-J> slines downward linewise, Not Join Lines (#14796)

https://vimhelp.org/motion.txt.html#CTRL-J

Release Notes:

- vim: Added `ctrl-j` as a new binding that's equivalent to `j`.

assets/keymaps/vim.json

@@ -10,6 +10,7 @@
       "backspace": "vim::Backspace",
       "j": "vim::Down",
       "down": "vim::Down",
+      "ctrl-j": "vim::Down",
       "enter": "vim::NextLineStart",
       "ctrl-m": "vim::NextLineStart",
       "+": "vim::NextLineStart",