vim: Implement <count>% motion (#25839)

Closes https://github.com/zed-industries/zed/discussions/25665

> Currently Zed is missing quite an useful Vim motion: <count>% (go to
{count} percentage in the file).
Description:
{count}% - Go to {count} percentage in the file, on the first non-blank
in the line linewise. To compute the new line number this formula is
used: ({count} * number-of-lines + 99) / 100 .
> [Link](https://neovim.io/doc/user/motion.html#N%25).

Release Notes:

- vim: Added `<count>%` motion

---------

Co-authored-by: Conrad Irwin <conrad@zed.dev>

assets/keymaps/vim.json

@@ -247,7 +247,8 @@
     "context": "VimControl && VimCount",
     "bindings": {
       "0": ["vim::Number", 0],
-      ":": "vim::CountCommand"
+      ":": "vim::CountCommand",
+      "%": "vim::GoToPercentage"
     }
   },
   {