vim: implement <space> in normal mode (#7011)

This fixes #6815 by implementing `<space>` in normal mode in Vim. Turns
out that `<space>` behaves like a reverse `<backspace>` (which we
already had): it goes to the right and, if at end of line, to the next
line.

That means I had to touch `movement::right`, which is used in a few
places, but it's documentation said that it would go to the next line,
which it did *not*. So I changed the behaviour.

But I would love another pair of eyes on this, because I don't want to
break non-Vim behaviour.

Release Notes:

- Added support for `<space>` in Vim normal mode: `<space>` goes to the
right and to next line if at end of line.
([#6815](https://github.com/zed-industries/zed/issues/6815)).

assets/keymaps/vim.json

@@ -31,6 +31,7 @@
       "up": "vim::Up",
       "l": "vim::Right",
       "right": "vim::Right",
+      "space": "vim::Space",
       "$": "vim::EndOfLine",
       "^": "vim::FirstNonWhitespace",
       "_": "vim::StartOfLineDownward",