Vim visual block mode

This isn't quite an exact emulation, as instead of using one selection
that is magically in "column mode", we emulate it with a bunch of zed
multi-selections (one per line).

I think this is better, as it requires fewer changes to the codebase,
and lets you see the impact of any changes immediately on all lines.

Fixes: zed-industries/community#984

assets/keymaps/vim.json

@@ -103,6 +103,7 @@
       ],
       "v": "vim::ToggleVisual",
       "shift-v": "vim::ToggleVisualLine",
+      "ctrl-v": "vim::ToggleVisualBlock",
       "*": "vim::MoveToNext",
       "#": "vim::MoveToPrev",
       "0": "vim::StartOfLine", // When no number operator present, use start of line motion